Strategies for water management in gravity sprinkle ...directory.umm.ac.id/Data Elmu/jurnal/A/Agricultural Water Management... · 3. Procedure The water available to users in an irrigation

Strategies for water management in gravity

sprinkle irrigation systems

A.A. Ramalana,*, R.W. Hillb

aAgricultural Engineering Department, Ahmadu Bello University, Zaria PMB 1044, NigeriabBiological and Irrigation Engineering Department, Utah State University, Logan, UT 84321-4105, USA

Accepted 17 March 1999

Abstract

The study examined various management strategies implementable for equitable allocation of

water among users in a gravity sprinkle irrigation system (GSIS). A computer model was developed

and employed to effect allocations under three scenarios or variations thereof, namely, on-demand,

rotation, and continuous supply. Historical streamflow records and climatic data for 3 years, 1979,

1986 and 1983, representing normal, dry and wet years, respectively, were used. Equitable water

distribution consideration took into account shares of stock for water held by each user in the

irrigation company. Allocations were proportionately determined in relation to the numeric strength

of the users' stock when the demand for water was constrained by the available streamflow. The

developed management strategies were applied to a real-life irrigation project, the Richmond

Irrigation Company at Richmond, UT. Pure on-demand strategy proved to be the most beneficial

project-wide, costs of augmentation notwithstanding. # 2000 Elsevier Science B.V. All rights

reserved.

Keywords: Gravity-sprinkle; On-demand; On-request; Strategies; Water-rights; Users

1. Introduction

The water source for the gravity sprinkle irrigation system provides water as well as the

energy for operating the system. No pumping is required to lift water and pressurize the

line as is the case in a conventional sprinkler system. The water source at certain

mountain top locations is several hundred meters in elevation above underlying farm

Agricultural Water Management 43 (2000) 51±74

* Corresponding author. Tel.: +234-069-50571; fax: +234-069-50563.E-mail address: [email protected]. (A.A. Ramalan)

0378-3774/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.

PII: S 0 3 7 8 - 3 7 7 4 ( 9 9 ) 0 0 0 4 6 - 3

lands. Thus, the elevation difference provides the pressure energy needed to deliver

irrigation water at the sprinkler nozzle. The system is fed directly with water from rivers

and creeks by the use of simple intake structures in open channels. Typically, these run-

of-the-river irrigation systems are characterized by absence of any flood detention or

storage structures.

In dry years, as a consequence of low snow pack on the mountain, streamflow in rivers

and creeks resulting from snow melt in the spring are deficient. Coupled with the high

crop water use in the summer, system performance diminishes. Equitable distribution of

water among users constitutes a serious water management problem to irrigation

companies1. Studies by Kaewkulaya (1980); Bishop and Long (1983); Khanjani and Bush

(1983); (Kim et al., 1983) examined methods for water allocation in open-ditch surface

irrigation systems. Information is, however, lacking on techniques for water allocation in

a gravity sprinkle irrigation system (GSIS).

A study to examine management strategies to adopt to effect such allocations was

undertaken. The nine strategies which evolved integrated with hydrological, soil and

climatic factors of the area, crop water use, and the amount of shares held by users in

company stock for water. As part of the study, a computer model to effect allocation was

developed. The model was tested using data from a real-life irrigation project in northern

Utah.

2. Objectives

The general objective of the study was to evolve techniques for improving water

management in gravity sprinkle irrigation systems. The specific objectives include

identifying and evaluating practical management strategies that could be used by GSIS

companies to aid water management. In addition, the study was to compare the attributes

of the various strategies and quantify the benefits associated with each.

3. Procedure

The water available to users in an irrigation company is a common commodity by

virtue of the legal provisions in water rights. Such waters may be either directly from a

diversion of surface source or from groundwater. The challenge for management of

mutual irrigation companies is to translate share of stock values into realistic water

allocations to users that are fair and equitable. Equity is viewed here in relation to

individuals getting their fair share of water. Time is an important element in the allocation

process because crop production occurs in sequential phenological phases that respond to

environmental changes. Water allocated to users must be in amounts and at times

1 Users constitute mutual irrigation companies, granted water-rights through an instrument of State Law todivert and distribute water to all share holders. The companies assess fees seasonally for each share of waterstock held by the user.

52 A.A. Ramalan, R.W. Hill / Agricultural Water Management 43 (2000) 51±74

reasonable to make the commodity usable and effective. Various management strategies,

or operational schemes, have been evolved as means to effect delivery to users. Table 1

gives a listing of the strategies used. A description of each strategy is presented below.

Except for the continuous supply strategy, the other strategies under on-demand and

rotation schemes are modified versions of the system used in open ditch irrigation

systems. Allocations based on selected strategy were implemented using the computer

model GSIS, developed as part of the study, which integrated soil, water and

environmental factors. The model and the submodels are illustrated in Fig. 1(a±d).

3.1. Descriptions of the management strategies used in the study

3.1.1. Pure on-demand (unlimited access to water by all users)

This is a strategy that seeks to allow all users, access to water at all times. The

implementation of this strategy is possible early in the season when an ample supply of

water is available and crop water requirements are relatively low. Irrigation durations are

similarly expected to be low. No intensive policing of the system is desirable.

The manner with which users may connect for service is expected to follow a random

pattern. The main constraint anticipated is with the capacity of the system hydraulics to

satisfy demand from users needing water within the same time frame.

3.1.2. Users placed on request

This strategy is a modified demand delivery system. It places some responsibility on

the user. It requires that he made known his request for water in advance of time of use to

allow a sufficient time lag for scheduling and balancing of deliveries and total system

flow rates by the project. Because this strategy allocates water subject to constraint in

Table 1A listing of the strategies used for the water allocation studies in gravity sprinkle irrigation system (GSIS) atRichmond, Utah

Strategy Composition

Pure on-demand

Unlimited access to water by all users fields treated as single entities.

Modified on-demand

Users' supply placed on request fields treated as single entities.

One-time delivery fields treated as single entities.

Rotation

Users grouped in rotation blocks contiguous fields as rotation blocks (three blocks)

Supply based on crop category each crop type forms a block (three blocks)

Min±max Ranking system range in shares of stock, each range constitutes a block (four blocks)

Systems-related strategy hand-move and wheel-move lateral system each constitute a block

(two blocks)

Low labour cost schemes Off-farm and on-farm users constitute a block each (two blocks)

Continuous supply

Continuous supply to users fields treated singly.

A.A. Ramalan, R.W. Hill / Agricultural Water Management 43 (2000) 51±74 53

Fig. 1. Flow diagram of gravity sprinkle irrigation system (GSIS) computer allocation model. (b) Flow diagram

of GSIS submodel HYDRA. (c) Flow diagram of GSIS soil-water budget submodel SWBAL. (d) Flow diagram

of GSIS submodel ATTR.


supply, the users' request may or may not be honoured at the requested time. Some

waiting may be imminent.

The strategy is particularly expedient during the latter part of the season when water

shortages are expected and irrigation amounts and/or frequencies are likely to be high.

Fig. 1. (Continued )


Users' requests are likely to be highest during times of critical crop growth stages. While

it may be possible to anticipate incoming requests for water, it may not be possible that

all requests made for deliveries at specific times will be met.



3.1.3. One-time delivery

This strategy considers supplying the user his allocation of water in a one-time

delivery. The one-time allocation will be estimated based on a quantity of water that

would refill the entire root zone of his crop in a one-time delivery. Users, through mutual

agreements, may trade water among themselves; management must be notified of such

deals in advance of trades.

3.1.4. Users grouped in rotation blocks

Users' holdings are grouped in rotation blocks. Each block is of a pre-determined size

(in hectares). Each user/field in the block is allowed access to a fixed continuous flow



amount and duration. The frequency is usually fixed, but may also be variable. Individual

users within each block irrigating at the same time are grouped as rotation units. Users

will have allocations figured out relative to their stock shares. Rotations are expected to

be rigid and all were to take their flows only when due. One rotation unit made up of one

or more blocks received water at any one time.

3.1.5. Supply users based on crop category

This strategy invokes the use of crop categories in allocation, taking into consideration

the economic benefits derivable from the array of crops irrigated. Ranking of users' crops

was in effect and allocations were to be along specific crop categories.

First to be allocated water were users who grew crops with the highest economic value,

and so on, to users who grow economically least-derivable-benefit crops, such as pasture.

The strategy is particularly well adapted to periods of diminishing streamflow.

3.1.6. Min±max ranking system

A ranking system was evolved whereby allocations were begun with users who hold

the highest number of shares of stock taking all the flow. This group of users was

followed by the next lower level of users and finally by users who held the least stocks.

When operating under deficit flow conditions, each user at any of the levels will have

water for a time period commensurate with his shares.

3.1.7. Systems-related strategy

This strategy relates to the two major gravity sprinkle irrigation systems, the hand-

move and the wheel-move lateral systems. Users were classified into the two groups

depending on the type of system each operates.

Allocations using this strategy were along system types, in other words, users operating

wheel moves will be expected to be irrigating at about the same time. The exact time

individuals within system groups are delivered water depended on the overall system

hydraulic analysis.

3.1.8. Low labour cost scheme

This management strategy is invoked whereby certain users are to receive their

allocations during the low labour cost hours. The scheme differentiates on-farm from off-

farm users. Under certain circumstances and locations, the cost of irrigation labour to the

farmer is lower during weekday first-shift hours compared to other shifts. The first shift is

between 8 a.m. and 4 p.m. The strategy is to make water available to the farmer during

the least labour cost hours.

Urban and non-agricultural irrigation stockholders who use water for lawn sprinkling

and backyard gardens will have access to water during the second and third shifts on

Saturdays and all three shifts on Sundays.

3.1.9. Continuous supply with monitoring

Continuous supply of water to users is to be maintained day or night as rationed to

users. Each user's ration is to be monitored by estimating and counting the number of

sprinklers he has operating continuously. The rations are available to the user whether or

not they are needed.


4. The computer model

The GSIS model is developed for use in mutual irrigation companies that operate under

gravity sprinkle systems. The model is comprised of four submodels, the first being the

main programme. The second, third and fourth are designated HYDRA, SWBAL and

ATTR submodels, respectively. The main programme serves the primary function of

controlling programme flow. It also carries out non-repetitive calculations. The submodel

HYDRA provides a work area in which routine calculations of crop water requirements

and the need for irrigation are performed. The submodel SWBAL maintains the soil water

balance sheet for all fields and the ATTR submodel evaluates the attributes of the various

strategies.

The model invokes a strategy to implement allocations, then estimates a field service

priority, and determines sprinkler discharges based on operating pressures at delivery

points. It also uses stored data on soils, crop and climate to compute crop evapotranspirat-

ion at various growth stages for maize (field corn), spring wheat, pasture and alfalfa from:

Et � KcKsEtr (1)

Where Et is the estimated crop evapotranspiration, Etr is the reference crop

evapotranspiration computed from the Jensen±Haise equation. Kc, the crop coefficient,

is the ratio of the actual crop evapotranspiration to reference crop evapotranspiration. The

value varies with crop variety or species, time of planting, and growth stages. It is

computed by the use of a phenology clock for each crop for the specific Julian day

depending on planting, effective cover and harvest dates. The mean crop coefficients, Kcm

used in the model were adapted from (Allen et al. (1988) based on tabular values

presented by Wright (1981, 1982); Doorenbos and Pruitt (1977) rev.). In the equation, Ks

is the soil moisture stress coefficient. For an irrigation system in which maximum Et is

allowed, Ks � 1. The coefficient varies with evaporative demand, soil moisture content,

soil and the crop. This study used the model proposed by (Hill et al., 1984).

The Jensen and Haise (1963) temperature-radiation equation for estimating Etr was

used to program a Datapod (Omnidata International DP 219) to accumulate the calculated

Etr on a daily basis. The data used including air temperature and solar radiation were

sampled at Newton, UT (the nearest weather station to Richmond). Thus,

Etr � CtCf�T ÿ Tx�Rs (2)

where Etr is the reference crop evapotranspiration (mm), T is the average daily

temperature (8C), Tx is the intercept on the temperature axis, Rs is incident solar radiation

(langleys/day), and Ct is a temperature coefficient. Values for Ct and Tx as 0.025 and ÿ3,

respectively, were reported in Hansen et al. (1979). Both factors are site-specific. For this

reason, they further gave a relationship for estimating Ct and Tx for other areas taking into

account altitude above mean sea level and saturation vapour pressures of water for the

warmest month of the year in the location. Allen et al. (1988) gave Ct (in English units) as

0.009 and 0.011, respectively, for valleys with extensive irrigated or wetland areas, and

for surroundings of predominantly arid lands. A fifth power polynomial equation was

derived that best fit the Datapod data relating Etr with day of year. The equation was used

in the model to estimate Etr for the Richmond site.


The hydraulic network submodel, HYDRA, simulates pressures in the distribution

main pipes, the laterals and sprinkler nozzles. The relationships given in Keller (1986)

were used to estimate average lateral pressures and nozzle discharges using data relating

to pipe type and size, friction head loss coefficient, and allowable head loss due to

elevation changes. It also provided for corrections for environmental and spray

conditions.

The submodel also estimates the allocation time for each user to irrigate with the

allotted volume of water, qt. The allotted volume was estimated by

qt � 2:8 a d (3)

where q is equal to field/user flow capacity (lps); t is equal to time to apply the irrigation

(h); a is equal to field area (ha) and d is equal to gross depth of irrigation (mm)

The time of water application, t, was related to infiltration rate of the soil in order to

prevent surface runoff and/or ponding. Therefore, the application rate, which is only

constrained by the soil infiltration rate, was calculated in the submodel using

i � 3600 qs

SmSl

(4)

where I is equal to application rate (mm/h); qs is equal to discharge per sprinkler (lps); Sm

equal to lateral spacing along mainline, and Sl is equal to sprinkler spacing along laterals

(m).

Once allocations are completed, programme execution is transferred to submodel

SWBAL, the soil water balance submodel.

The submodel SWBAL maintains a soil±water budget. This is a daily book-keeping

process of where water is in the soil/crop environment. The soil±water budget equation is

SWSn � SWSnÿ1 � Prn � IRn ÿ Etn ÿ DRn (5)

where SWSn is the soil water content at the end of the present day, SWSnÿ1 was the soil

water content the day before, Prn, Irn, Etn, and Drn are, respectively, the precipitation

amount, irrigation depth, actual crop evapotranspiration and drainage loss for the present

day. Drainage occurred only if SWSn for any day exceeded the field capacity storage of

the soil. The present day n subscript refers to a specific day of the year. In the

development of the submodel, assumptions were made that the soils within the layers

found were homogeneous, infiltration occurred at the same rate in fields, there was no

upward movement of water into the root zone, all farmers (users) used the correct nozzle

sizes on their sprinklers at all times, and the management allowed depletion was 50% of

available water in the root zone (Ramalan, 1988).

As soil water storage capacity is tied to rooting depth, the model uses a root growth

function to estimate crop root depth in relation to time. The function assumes that root

growth progresses linearly from a minimum root depth at the time of planting (or begin

growth in the case of alfalfa) to a maximum root depth value at crop full cover.

Crop yield is estimated by the model using the method of Hill et al. (1984) based on

the ratio of actual to potential transpiration for each growth stage. Influence of

each growth stage has an effect on the estimate because of the multiplicative nature of

the relationship. Crop growth progression through a number of growth stages, was


estimated using a phenology clock based on growing degree-days proposed by Gilmore

and Rogers (1958).

The submodel ATTR is invoked to evaluate attributes of the various strategies,

including costs, returns, and benefits. In addition, waiting time with respect to elapsed

time before a user receives service, and equitability indices are estimated in this

submodel. The monetary benefit was simply estimated from total returns from sale of

produce minus cost of production. Returns were estimated as

Returns � Price�per unit weight� � Yield

ha� BYP (6)

where unit weight was based on tonnage or kg. In the case of pasture the unit weight was

replaced by animal unit month (AUM), as 1 AUM � 360 kg DM (Scarnecchia and

Kothmann, 1982). Yield was based on production per unit area, tonne/ha. BYP was the

revenue from sale of bye-products, such as wheat straw. All the costs toward production

were partitioned as yield-dependent, constant or fixed, area-dependent, or capital costs.

Thus on-farm total production cost per hectare was estimated as

TC � Cy � Cc � Ca � Ck (7)

where TC is equal to total cost/ha; Cy to yield-dependent costs such as of fertilizers and

fertilizing; swathing and baling for alfalfa; and clipping for pasture. Cc is equal to

constant costs, such as the cost associated with land tax; Ca to area-dependent costs, such

as water assessment and irrigating costs. Ck is equal to capital costs, such as costs

associated with machinery and tools. Its value depends on the investment cost of the item,

interest rate and pay-back period on loan amount.

The submodel ATTR estimated annual cost of capital items using capital recovery

factor CRF, thus,

Annual cost � investment cost� CRF (8)

where

CRF � i�1� i�n�1� i�n ÿ 1

i is equal to annual interest rate expressed in decimal and n is equal to pay-back period for

the loan sum in years.

The input data for calculations in this routine were stored in ECON.DAT for the four

crop enterprises in the study.

5. Application of the model

5.1. The site

The Richmond Irrigation Company (RIC) project was the site of the study conducted in

the summer of 1986. The project, located in Richmond (418430N, 1118470W) in north

Cache County, UT, as shown in Fig. 2, was developed in stages following the company's


formation in the year 1860 by a group of farmers (users). The sole purpose of the

company was to effect distribution of water among members for irrigation and municipal

uses. The source water is by direct diversion from High Creek, near Richmond. The area

of the watershed contributing flow in the creek is 26.1 km2. From historical records

(1944±1985), the mean discharge at the main diversion gauging station is about 83 lps.

The extreme maximum and extreme minimum discharges ever recorded were 14,000 lps

and 70 lps, respectively.

The users who are the stockholders in the company, own and operate farms within the

project. The profile of stockholders showed that there were 457 of them in the register

who collectively owned 6067 shares. The maximum, average and minimum number of

Fig. 2. Location map for Richmond Irrigation Company.


shares held by stockholders are 229, 13.28, and 1, respectively, and all were acquired as

sprinkle irrigation stock. The farms were flood-irrigated until 1970, when construction of

the gravity sprinkle irrigation system was begun (USDA ± Soil Conservation Service,

Logan, 1974). The RIC had a total of 10,000 ha for which water-rights were granted by

the Utah Division of Water Resources through the Kimball Decree Awards #38. The

priority date for the first right to the company was issued on 1 May 1860. Three other

irrigation companies in the vicinity which also have water-rights on the High Creek are

Coveville, Webster, and Mt. Home.

The interpretation of these awards (Ramalan, 1988) which are in three parts gives RIC

the right to divert 78.4% of the flow in High Creek for all flows between 141.2 lps (5 cfs)

and 1412 lps (50 cfs). If the flow in the creek is greater than 1412 lps (50 cfs) and up to

2613 lps (92.2 cfs), RIC has an additional award to divert 20.2% of flow that is in excess

of 1412 lps (50 cfs). At this 2613 lps (92.2 cfs) flow, which is regarded as maximum ditch

flow, the company by virtue of the two awards has rights to divert a total of 1338 lps

(47.2 cfs). The third award comes into effect if the flow in the creek exceeds 2613 lps

(92.2 cfs). The third award allows the company an additional 20.2% subject to an upper

ceiling of 1984 lps (70 cfs). At the time of study the RIC had not operated any ground

water rights.

5.2. Pumping

Four electrical deep well turbine pumps exist within the project. The pumps with rated

discharges of 60 lps (900 gpm) at heads 80 m (260 ft) were to pump from 30 cm diameter,

98.8 m deep wells. The pumps were located in a fenced pump house in sector four close

to the Logan±Richmond highway. The location was a few hundred meters from the pipe

intake structure across the road. The pumps feed the project mainline which in turn feed

distribution lines servicing different farmers' fields. The pumps were not designed to

serve individual fields rather they feed into the common pipelines arranged in branching

hydraulic network. The study took cognisance of these facilities to include the possibility

of need for flow augmentation by use of ground water.

5.3. Procedure for estimating augmentation and costs

The general procedure used to estimate augmentation requirement and its costs,

especially in relation to strategies 1 and 8 is summarized below. The energy type was

electricity and a Utah Power and Light Company (UPLC), schedule of charges used.

Augmentation requirement was estimated

Qaug � DQJd ÿ HQJd �if Qaug > 0� (9)

and

Vaug � 8:64� 10ÿ3Qaug (10)

where Qaug is equal to the required flow augmentation in lps; DQJd is equal to the demand

flow on a particular Julian date in lps; HQJd equal to historical streamflow for the

particular Julian date in lps; Vaug is equal to volume of augmentation in ha m.


It was anticipated by the project designers that augmentation, if needed, would not

exceed 226.7 lps (8 cfs) even in dry years. Such need is to be met by the output from the

four pumps installed. Each pump was rated at 56.67 lps (2 cfs) and powered by 75 hp

motors connected to three-phase, 415 V power source. The number of pumps needed to

supply augmentation on any day was estimated by (rounding up to next integer)

PUMPN � Qaug

56:67(11)

i.e., one pump if 0 < Qaug � 56:67 lps; two pumps if 56:67 lps < Qaug � 113:33 lps;

three pumps if 113:34 lps < Qaug � 170:00 lPs; and four pumps if 170:00 lps < Qaug

� 226:66 lps; where PUMPN is equal to number of pumps operating.

The electric power requirement for the pumps to supply the augmentation flow was

estimated

Power � Qaugh

56:6 Ep

(12)

where Power is equal to required power of electric motor in kW; h to pumping depth in

m; Ep is equal to pump efficiency (expressed in decimal) and Qaug as previously defined.

The energy requirement was calculated as

Energy � Power� Time (13)

where Energy and Time were measured in kW h and h, respectively. Both power and

energy were tallied to obtain monthly and seasonal totals. Monthly power and energy

costs were estimated using option A of the UPLC schedule. Monthly bills for

augmentation were estimated in the model by

MNBILL � FXCOST� SERVCHG� �MISCOST� DEMCOST � ENECOST�mn

(14)

where MNBILL is equal to monthly bill in US$; FXCOST is equal to fixed cost

representing amortized costs for wells, motors and pumps, and shelter based 30, 20, and

10 year lives, respectively, at 12% interest rate. All cost details were obtained from the

local equipment dealer. SERVCHG is equal to service charge per connection by the

Utility company. MISCOST is equal to miscellaneous costs, which are associated with

attendance, lubrication and repairs on per 100 h basis for the month. DEMCOST is equal

to UPLC monthly power demand cost based on $8.04/kW, first 100 kW; and $5.33/kW all

additional kW; ENECOST is equal to UPLC monthly energy cost based on 10.0345

cents/kW h first 100 kW h per kW; 5.3803 cents/kW h next 25,000 kW h; and 3.5702

cents/kW h all additional kW h, and mn is subscript referring to the specific month.

Discounts were applied to the monthly bill to obtain the net monthly bill. The discounts

were with respect to term and voltage. Term discounts were not in operation at the time of

the study. Voltage discounts were rated based on power demand and were contingent

upon the customer providing and maintaining the transformer and necessary equipment.

The voltage discount rates operated by UPLC are: $1.17/kW, first 100 kW of power; and

$0.62/kW, all additional kW.


Seasonal bill is a summation of monthly net bill minus off-season costs. The off-season

costs are equipment costs for which payment must be made during and off-irrigation

season. These data were entered in a data file, AUGMENT.DAT for use in the model.

5.4. Farm and field characteristics

The RIC project is in four sectors. Each sector is served by an intake structure that

feeds water into the mainline for the fields in the sector. The data collected for this study

were gathered in the fourth sector which has an area of 251 ha.

For at least the purpose of this study, a field was considered a piece of land with unique

crop, soil, and physical characteristics. The field was, therefore, expected to consist of

one crop on essentially one soil type. The field had physical boundaries based on either

soil or crop differentiation. One or more fields constituted a farm. Records of field sizes

obtained through direct interview with the farmers varied from 2 to 32 ha with a mean of

11 ha.

Soil types defined by textural classification given in the soil survey data of the area

showed that Trenton silty clay loam was the most common. About 70% of the area under

this soil type is cropped with irrigated crops such as alfalfa, small grains, sugar beet, and

pasture. This soil and others elsewhere on the site are moderately well drained though

slowly permeable. Runoff is low and hazards to erosion slight. Water-table depth is

commonly more than 100 cm.

Crops in the project fall within the categories of irrigated or dryland commercial crops.

Their distribution among fields, however, varied with climatic and soil conditions,

economics and farmers preferences. For the 1986 crop year, aerial distribution of crops

among fields in the study area was as follows: alfalfa, 65%; small grain, 13%; silage corn,

9%; and pasture, 13% on land areas of 181, 14, 24, and 32 ha, respectively. About 177 ha,

out of the 251 ha, were on Trenton silty clay loam and Nibley silty clay loam soils.

Water for irrigation was distributed through a branching pipe network. The irrigation

systems in the farms are sprinklers of either hand-move or the periodic wheel-move

systems. Water channelled from the creek in open ditches was fed to the pipe inlet at

canal turnout about 60 m in elevation above the lowest farm delivery points. Most

delivery points were located about 52 m below the elevation of canal turnout. Excess

pressures not used in the pipeline were dissipated at pressure relief valves strategically

located within the network. Pipe sizes of standard lengths of 6 m made from welded steel

or asbestos cement and fittings were joined and buried to constitute the network.

At delivery points, the network terminated to 100 mm stand valves equidistantly

spaced on laterals along fence lines of individuals' properties. Valve openers facilitated

release of water into individual sprinkler systems.

5.5. The model implementation

The computer model was invoked for each of the nine allocation strategies and for the

3 years representing dry (1979), normal (1986) and wet (1983) years. These

representative years were selected from the water-year records of High Creek (October

1978 through September 1987). For these years, the available streamflow diversions to


the study area, as allowed for by the water rights, varied from 14 lps to a maximum of

178 lps. Daily streamflow data adjusted with water-right awards factor (as a percentage)

represents the allowable diversion due to RIC. These daily estimates were stored in an

array variable HQ in data files CLIM.DRY, CLIM.NOM and CLIM.WET. They were

read for each computer run. Recorded precipitation are also included on a daily basis as

part of the input files. Each computer run called the relevant subroutine after having

previously opened and read all input data files. All the calculations carried out in the sub-

routines are written in output data files before exiting to the main programme.

6. Results and discussions

For the test years, with allocation on demand, the study showed that it was infeasible to

operate the strategy for allocation without augmentation throughout the growing season.

This is evidently so because demand flow rates on the system outstripped supply in some

instances, as depicted by the hydrographs in Fig. 3 particularly in the flow receding

region where demand-to-supply ratio exceeded 8 : 1. In the normal and wet years, it was

possible to operate on-demand from day of year 101 through Day 135 and Day 175,

respectively. Within these periods demand flows could easily be satisfied by supply. But

the supply hydrograph for the dry year showed significant deficit as to make allocations

on-demand inoperative. Scanty precipitation coupled with low stream flows preclude

operating on-demand except intermittently for some 10 days throughout the growing

season. The model showed that in addition at least one pump needed to be operated

continuously.

Beyond those periods when on-demand allocations were feasible in the normal and wet

year, constraints in delivery were inevitable. From these days onward, on-demand was

viable only to the extent that water supply was augmented through conjunctive water use

by tapping groundwater sources. The total costs for augmentation in order to continue to

operate on-demand were $44,679, $96,257, and $144,735 in wet, normal and dry years

respectively. A substantial proportion of this cost was for energy for pumping. In the only

alternative, as would be the case for most run-of-the-river GSIS with no groundwater

rights, the schedule of allocations has to change to a less flexible system such as rotation

or continuous supply.

The data on seasonal crop water use for the test years for the three strategies are given

in Table 2. The average seasonal actual crop Et's were 760, 510, 580 and 710 mm for

alfalfa, corn, small grain, and pasture, respectively. The number and net depths of

irrigation were highest under the continuous supply strategy. Drainage, estimated by the

model as the component of applied water or rain that had percolated below root zones,

was least with the on-demand mode, where values exceeded 25 mm per season. These

drainage amounts were mainly in corn and grain fields. The seasonal rainfall received in

alfalfa and pasture fields was about 280 mm with about 178 mm on the corn and small

grain fields.

Benefits accrued to individuals were calculated through the itemization of costs and

returns. Crop yield using the technique of Hill et al. (1984) provided vital data for

estimating the financial returns. The fields under pastures sustained financial loss. When


Fig. 3. Available and demand flow hydrographs for irrigation on-demand, precipitation, number of pumps for supply augmentation, and number of users irrigating based

on Dry Year (1979) data, RIC.

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Table 2Summary of water use (seasonal totals) by field/user during a normal year with the on-demand, rotation, and continuous supply strategies invoked

Field/

user code

Crop Area

(ha)

Rain

(mm)

On-demand Rotation Continuous supply

No.

of

irrg

Net

irrg

(mm)

Drain

(mm)

RT-Z

depl

(mm)

Crop

Et

(mm)

No.

of

irrg

Net

irrg

(mm)

Drain

(mm)

RT-Z

depl

(mm)

Crop

Et

(mm)

No.

of

irrg

Net

irrg

(mm)

Drain

(mm)

RT-Z

depl

(mm)

Crop

Et

(mm)

CARI430 ALFALFA 16 281.9 3 408.9 0.0 61.0 754.4 10 449.6 147.3 170.2 754.4 16 457.2 137.2 152.4 754.4

CAR1593 ALFALFA 16 281.9 3 414.0 5.1 61.0 751.8 10 574.0 236.2 132.1 751.8 16 586.7 223.5 106.7 751.8

CARI688 S/GRAIN 16 188.0 6 464.8 83.8 15.2 584.2 10 398.8 243.8 157.5 500.4 13 419.1 248.9 160.0 518.2

CARI836 PASTURE 20 289.6 7 495.3 55.9 2.5 731.5 10 355.6 162.6 127.0 609.6 16 363.2 152.4 127.0 627.4

CARI525 PASTURE 4 281.9 6 416.6 45.7 66.0 718.8 10 566.4 279.4 132.1 701.0 16 574.0 266.7 129.5 718.8

CARI750 S/GRAIN 8 200.7 6 472.4 91.4 2.5 584.2 10 317.5 208.3 154.9 464.8 13 325.1 203.2 157.5 482.6

CARI142 PASTURE 8 289.6 7 487.7 58.4 15.2 734.1 10 599.4 297.2 119.4 708.7 16 599.4 287.0 124.5 729.0

CARI950 ALFALFA 32 281.9 2 436.9 0.0 ÿ27.9 690.9 10 66.0 0.0 208.3 558.8 16 68.6 0.0 208.3 558.8

CARI011 ALFALFA 6 281.9 3 408.9 2.5 63.5 751.8 10 571.5 221.0 119.4 751.81 16 581.7 218.4 106.7 751.8

CARI066 ALFALFA 12 281.9 3 403.9 10.2 76.2 751.8 10 569.0 226.1 129.5 751.8 16 584.2 228.6 114.3 751.8

CARI122 ALFALFA 6 289.6 3 406.4 15.2 68.6 751.8 11 594.4 236.2 101.6 751.8 16 599.4 233.7 91.4 751.8

CARI474 ALFALFA 12 289.6 3 424.2 17.8 53.3 751.8 11 594.4 238.8 104.1 751.8 16 609.6 238.8 88.9 751.8

CARI299 ALFALFA 12 289.6 3 408.9 7.6 61.0 751.8 10 576.6 228.6 114.3 751.8 16 581.7 221.0 101.6 751.8

CARI869 ALFALFA 14 281.9 3 408.9 2.5 63.5 751.8 10 294.6 78.7 208.3 706.1 16 302.3 76.2 208.3 716.3

CARI958 CORN/SL 10 172.7 3 317.5 5.1 33.0 518.2 10 452.1 137.2 12.7 502.9 14 533.4 182.9 7.6 528.3

CARI481 ALFALFA 12 281.9 4 510.5 17.8 ÿ22.9 751.8 10 444.5 149.9 177.8 751.8 16 411.5 129.5 188.0 751.8

CARI986 S/GRAIN 4 175.3 6 467.4 76.2 15.2 581.7 10 825.5 538.5 119.4 581.7 13 777.2 487.7 119.4 581.7

CARI399 CORN/SL 4 172.7 4 396.2 50.8 5.1 525.8 9 538.5 200.7 5.1 515.6 14 510.5 175.3 5.1 513.1

CARI640 ALFALFA 4 281.9 3 406.9 0.0 68.6 756.9 9 388.6 94.0 177.8 756.9 16 363.2 81.3 193.0 756.9

CARI059 ALFALFA 10 292.1 3 411.5 22.9 71.1 749.3 10 289.6 88.9 193.0 685.8 16 279.4 88.9 193.0 675.6

CARI618 ALFALFA 20 297.2 2 340.4 0.0 119.4 756.9 9 396.2 106.7 170.2 756.9 16 370.8 91.4 180.3 756.9

CARI181 ALFALFA 2.8 281.9 3 403.9 2.5 68.6 754.4 9 490.2 165.1 144.8 754.4 16 457.2 139.7 152.4 754.4

CARI714 ALFALFA 2 289.6 3 401.3 12.7 71.1 751.8 10 619.8 261.6 101.6 751.8 16 581.7 223.5 101.6 751.8

Note: NET IRRG: applied water that infiltrated the soil surface; RT-Z DEPL: root zone depletion.

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crops were not water-stressed, average benefit to users who engaged in the following

crop enterprises were as follows: alfalfa, $746/ha ($44/tonne); silage corn, $832/ha

($15/tonne); small grains, $281/ha ($474/tonne); and pasture, ÿ$84/ha (ÿ$14/tonne).

With constrained allocations, results showed that benefits accruing to users are lower

and indeed some alfalfa fields returned negative benefits in the dry year. The

aggregated benefits resulting from the use of each strategy are included in Table 3 for

the test years.

The relative ratios of user allocations and stock for water held in the company were

compared to draw inferences regarding equitability of allocations among users. The

results given in Table 2 for the normal year using the strategies showed that some users

received more or less water than their shares of stock would otherwise entitle them to. In

the on-demand mode no restrictions were imposed, each field received as much water as

it needed. Those fields/users that got far less relative to their shares had far more shares

than they needed. While no absolute agreement between the ratios was expected and

obtained with on-demand, the non-flexible strategies attempted to ration water based on

shares of stock.

Waiting time or stress-days for this study was calculated based on number of days since

50% available soil moisture storage was depleted. It varied with strategy used and the test

year. For the on-demand strategy waiting time in the dry, normal and wet years were 11,

7, and 5 days, respectively. Waiting time for other strategies were also estimated. The

durations varied between 28 and 101 days. Waiting time with continuous supply was

shortest, because users received water on a daily basis. Waiting time for all strategies

were longest during the dry year and shortest in the wet year. There was evidently a

relationship between waiting time and crop yield achieved. It appeared that the longer the

waiting time with a given strategy, the more the individual fields are stressed and the less

the crop yields and potential benefits to users.

The resource utilization ratio (RUR), provided a measure of how closely total

allocations to users related to available water resources at the disposal of the company. In

the dry season, RUR amounted to 1.84, thus users required 84% more water in volumetric

terms on a seasonal basis than they had water rights for in order to operate on-demand.

The specific times to inject additional water can be seen on observation of demand and

supply hydrographs, given as Fig. 3. In normal and wet years, the ratios potrayed a

standpoint of sufficiency in meeting seasonal demands; this is to be realized only if

assumption of trading surplus water at the beginning of the season holds. If not,

augmentation from groundwater had to be in effect. To match allocations closely with

what was available, the ratios had to be less than 1.0. The data on Table 3 showed that

RUR ranged 0.32 and 0.94 for the other non-flexible strategies. With one-time delivery

strategy, where only one heavy irrigation was given to refill the root zones, in the normal

year just about 35% of the streamflow was utilized. Augmentation was needed only if

streamflow was low to satisfy the commitment for the one-time delivery.

The data in Table 4 represent the average of five runs and summarize the results of the

response of the irrigation system to all of the allocation strategies in what are described as

`attributes' of the management strategies. On-demand strategy returned the highest

benefit to users only if augmentation is at no cost (such as when water is traded). But, if it

has to be paid for, on-demand in spite of its flexibility is a lower revenue-earning option


Table 3A summary of attributes for management strategies implemented in the dry, normal, and wet years (1979, 1986and 1983), respectively

Strategy Season

type

Benefit

$

Wait time

days

Water supplieda RURb

Stream Augm.

On demand

Unlimited access to waterc dry 143464 11 80.9 103.0 1.84

norm. 153961 7 72.0 67.2 1.00

wet 154331 5 87.2 28.9 0.79

Unlimited access to waterd dry ÿ5214 11 80.7 103.0 1.84

norm. 51111 7 74.3 67.2 1.02

wet 105509 5 89.0 28.9 0.80

Modified demand

User on request dry 64268 74 90.7 0 0.91

norm. 89183 58 72.0 0 0.52

wet 136231 34 92.6 0 0.63

One time deliverye dry 5294 101 46.6 24.3 0.71

norm. 84175 67 47.8 0.85 0.35

wet 117441 65 47.4 0.13 0.63

Rotation

Users in rotation blocks dry 34203 79 84.0 0 0.84

norm. 110330 38 126.5 0 0.91

wet 127400 29 130.4 0 0.89

Among crop categories dry 19651 91 83.5 0 0.83

norm. 105760 43 123.8 0 0.89

wet 125414 37 127.4 0 0.87

Stock ranks dry 28757 81 86.0 0 0.86

norm. 109227 40 127.6 0 0.92

wet 128314 35 132.1 0 0.90

System types dry 28280 83 82.4 0 0.82

norm. 106015 42 126.5 0 0.91

wet 125889 38 131.0 0 0.89

Labor cost scheme dry 21426 90 84.6 0 0.85

norm. 103073 43 125.7 0 0.90

wet 123639 38 131.3 0 0.90

Continuous supply

Continuous supply dry 23841 80 89.6 0 0.89

norm. 102651 35 130.1 0 0.94

wet 117763 28 135.6 0 0.93

a Water supplied, measured in ha m.b Resource utilization ratio (RUR), defined as the ratio of water supplied (stream plus augmentation) and

allocated to users to streamflow resource available to project for which water-right is granted.c Assumes unlimited access to water, with no cost supply augmentation during times when flow demand

exceeds available supply hydrographs.d Augmentation costs included were $144,735, $96,257, and $44,679 for dry, normal, and wet years,

respectively.e Augmentation costs included were $38,405, $5,665, and $4,479 for dry, normal, and wet years,

respectively.


Table 4Field/user stock relative to allocation of water received (and expressed in percentage) using on-demand strategy in a normal year

Field/user

code

User

stock (%)

User as a ratio of

company stock (%)


User

allocation

(%)

User as a

ratio of all

allocations

(ha m)

User

allocation

(%)

User allocation

as a ratio of

all allocations

(ha m)

User

allocation

(%)

User allocation

as a ratio of

all allocations

CARI430 40 7.1 8.4 6 9.4 7.5 9.6 7.4

CARI593 51 9.1 8.5 6 12.1 9.5 12.3 9.5

CARI688 40 7.1 10.0 7.1 8.4 6.6 8.9 6.8

CARI836 39 7.0 12.6 8.9 9.2 7.3 9.5 7.3

CARI525 12.5 2.2 2.2 1.6 3.0 2.3 3.0 2.3

CARI750 15 2.7 5.2 3.7 3.2 2.6 3.4 2.6

CARI142 26 4.6 5.1 3.6 6.3 5 6.4 4.9

CARI950 12 2.11 9.2 13.6 2.4 1.9 2.6 2

CARI011 19 3.4 3.2 2.3 4.5 3.6 4.6 3.5

CARI066 38 6.8 6.4 4.5 8.9 7 9.1 7

CARI122 19 3.4 3.1 2.2 4.7 3.7 4.7 3.6

CARI474 38 6.8 6.9 4.9 9.1 7.2 9.6 7.4

CARI299 38 6.8 6.8 4.8 8.4 6.7 9.2 7

CARI869 23 4.1 7.3 5.2 4.5 3.6 5.2 4

CARI958 20 3.6 4.1 2.9 5.3 4.2 6.6 5.1

CARI481 27 4.8 8.2 5.8 5.9 4.6 5.4 4.1

CARI986 19 3.4 2.5 1.8 4.1 3.3 3.9 3

CARI399 8 1.4 2.1 1.5 3.3 2.6 2.6 2

CARI640 8 1.4 2.2 1.6 1.9 1.5 1.8 1.4

CARI059 14 2.5 5.5 3.9 2.5 2 2.6 2

CARI618 41 7.3 9.3 6.6 8.4 4.7 6.0 4.6

CARI181 7 1.2 1.5 1 1.8 4.7 1.7 1.3

CARI714 6.3 1.1 1.1 0.8 1.6 1.3 1.5 1.2

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Table 4 (Continued )

Field/user

code

User

stock (%)

User as a ratio of

company stock (%)


User

allocation

(%)

User as a

ratio of all

allocations

(ha m)

User

allocation

(%)

User allocation

as a ratio of

all allocations

(ha m)

User

allocation

(%)

User allocation

as a ratio of

all allocations

Month Aug. (ha m) Vol. (h) Aug. (kW h) Hour ($) Energy ($) Costs ($) Discounts ($) Netbill ($)

April 0 0 0 713.75 0 713.75

May 0 0 0 713.75 0 713.75

June 9.46 432 19336 14502.15 554.52 13947.64

July 21.85 744 44663 31799.81 1208.81 30591.01

August 32.83 744 67117 46976.20 1788.87 45187.34

September 3.02 216 6177 5317.71 214.56 5103.15

Total vol. of

augmentation

(ha m)

67.2 0 0 0

Total aug. cost $96,257.00 0 0

Total benefit

to project

$51,111.00 $110,330.00 $102,651,00

Average waiting

time (days)

7 38 35

Total water

allocated (ha m)

141.5 126.4 130.2

Resource

utilization ratio

0.94 1.02 0.91

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to the project. On-request, because of its bias toward the more profitable crops and

enhanced productivity, returned higher benefits to the project relative to the amount of

water supplied. The higher productivity was, however, at the expense of equity. One-time

delivery performed almost as well as on-request in the normal year. The one heavy

dosage of irrigation given to refill the root zone was not beneficial to grain crops in

particular as some fields failed, more so in the dry year. Continuous supply performed no

better than any of the modified rotation strategies; it had slightly lower average waiting

time in the dry year.

7. Conclusions

The study addressed a felt need in run-of-river gravity sprinkle irrigation system. The

strategies developed for managing water allocations using the GSIS computer model

provides a valuable management-decision tool for simulation as well as operation.

In implementing allocations in normal and wet years, on-demand strategy is an obvious

choice since emphasis are on higher monetary benefit to the project and less waiting time.

No attempt should be made to operate on-demand in the dry years. At these times, it is

incumbent on the project to deliver water to all users to satisfy the legal provision of the

water-rights. To such end, continuous supply or rotation strategies facilitate allocations

more equitably, though at the expense of achieving high productivity. Augmentation in a

dry year is not advisable because it results in financial loss unless cheaper energy for

pumping is available.

Acknowledgements

The work reported here enjoyed partial support from the Utah Division of Water

Resources to which we are gratefully appreciative. Gratitude is also expressed to the

President and several members of the Richmond Irrigation Company for granting

unlimited access to their lands for data collection and to the staff of USDA ± NRCS

(formerly SCS) Cache Valley office in Logan for assisting us with construction drawings

of the pipe network. Finally, the skill and patience of Becky Andrus for finalizing the

figures and equations and Peggy Shumway for retyping the revised manuscript is greatly

appreciated.

References

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for Irrigation and Drainage in the 21st Century. Specialty Conference of the Irrigation and Drainage

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Bishop, A.A., Long, A.K., 1983. Irrigation water delivery for equity between user. J. Irrig. Drainage Eng. ASCE,

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Gilmore Jr., E.C., Rogers, J.S., 1958. Heat units as a method of measuring maturity in corn. Agron. J. 50, 611±

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Hansen, Vaughn E., Israelsen, Orson W., Stringham, Glen, E., 1979. Irrigation Principles and Practices, fourth

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Hill, R.W., Hanks, R. John., Wright, James, L., 1984. Crop yield models adapted to irrigation scheduling

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Khanjani, M.J., Bush, J.R., 1983. Optimal irrigation distribution systems with internal storage, transaction. Am.

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Kim, S., Busch, J.R., Yoo, K.H., 1983. Predicting daily irrigation project diversions. Research Technical

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Ramalan, A.A., 1988, Management Strategies for Gravity Sprinkle irrigation Systems, Ph.D. Dissertation. Utah

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Scarnecchia, David L., Kothmann, M.M., 1982. A dynamic approach to grazing management terminology. J.

Range Manage. 35(2), 262±264.

Wright, J.L., 1981. Crop coefficients for estimates of daily crop evapotranspiration. Irrigation Scheduling for

Water and Energy Conservation in the 80s, ASAE. Proceedings of 1981 Irrigation Scheduling Conference.

Chicago, IL, pp. 23±81.

Wright, J.L., 1982. New evapotranspiration crop coefficients. J. Irrig. Drainage Division, ASCE 108, 57±74.


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