Planning and management of irrigation water

Planning and management of irrigation water

Pieter van HeerdenWith material from:

Prof Sue Walker, Dr Dirk Raes, Mr Felix Reinders

Introduction

Background to irrigation water planning

Need to know about:

• Crops and their characteristics

• Soils and their characteristics

• Climate / weather of the area

• Engineering considerations

• Economic considerations

Crops

Growing characteristics• Suitability for climate• Suitability for soil • Planting date• Growing period• Height• Density / foliage cover• Rooting characteristics• Soil water extraction• Adaptation to water stress situation

Soils

• Texture

• Structure

• Depth

• Chemistry

• Infiltration rate

Climate / weather

• Average, maximum and minimum temperatures

• Humidity

• Frost

• Wind

• Sunshine / radiation

• Rain, season

• Hail, storms

Engineering

• Suitable and affordable systems

• Suitable for crop

• Suitable for soil

• Losses between pump and rooting area

Economic considerations

• Markets

• Funding

• Profit

• Availability of input material

Irrigation scheduling

Irrigation scheduling

= Getting the right amount of water to a crop at the right time

This means that you should:• Know your crop and its characteristics• Know your weather and its influence crop water requirement• Know your soils, their water holding capacities and their

limitations and or problems• Know your irrigation systems, their applications and

limitations• Estimate expected irrigation requirements within the scope

of source, system, crop, soil management limitations• Do real-time irrigation management and adapt your plan if

necessary

Soil, Crop, Atmosphere Continuum

Simplified soil-crop-atmosphere continuum

• Water moves from soil to crop roots: low water content and high salinity changes osmotic potential: less water for crop.

• Water moves through roots to stem: movement restricted: high atmospheric demand cause temporary wilting.

• Water moves through stem to leaves: as above.

• Water vapour from leaves during transpiration; speed depends on:– Weather situation– Soil water content– Crop characteristics

Determination of crop water use

• Direct measurement– Not easy or cheap– Used mainly for management and less so for

planning

• Relative to a known quantity– A-pan and other evaporation pan approaches– Calculation approach

• Penman-Monteith and others

Penman-Monteith

• Use a reference crop to which which water use of other crops are related

• Definition

A hypothetical crop with an assumed height of A hypothetical crop with an assumed height of 0.12 m, having a surface resistance of 70 s m0.12 m, having a surface resistance of 70 s m -1-1 and an albedo of 0.23, closely resembling the and an albedo of 0.23, closely resembling the evaporation of an extensive surface of green evaporation of an extensive surface of green grass on uniform height, actively growing and grass on uniform height, actively growing and adequately watered.adequately watered.

Relating crop water requirement to reference

Relating crop water requirement to reference

Relating crop water requirement to reference

Relating crop water requirement (ETc) to reference evapotranspiration (ET0)

Relating crop water requirement (ETc) to reference evapotranspiration (ET0)

• ET0 : reference evapo- transpiration

• ETc: crop evapo- transpiration

• Kc: crop coefficient = relation between crop water use and reference water use

CROP COEFFICIENT APPROACH

• single crop coefficient – used for applications – to irrigation planning, design, and management.

• used for calculating crop evapotranspiration under standard conditions, ETc.

• standard conditions refer to – crops grown in large fields under excellent agronomic – soil water conditions.

• crop evapotranspiration ETc different from reference evapotranspiration, ETo, – ground cover, canopy properties & aerodynamic resistance of

the crop are different from grass. – effects of characteristics that distinguish field crops from grass

are integrated into the crop coefficient, Kc.

Effect of weather

• On evapotranspiration in ET0

• Effects of cropped surface as differ from reference surface are – integrated into the crop coefficient,

ETc = Kc.ET0

• Effects of soil water stress Ks

– multiplying Kc.Ks

ETc = Kc.Ks.ET0

ETc = Kc.ET0

• ETc is the crop evapotranspiration [mm d-1]Kc is the crop coefficient [dimensionless]ET0 is the reference evapotranspiration [mm d-1]

• most weather effects in ET0 estimate. ET0 = index of climatic demand, • Kc varies specific crop characteristics

– only to a limited extent with climate.

transfer Kc values between locations & climates reason for global acceptance & usefulness Kc

Kc factors developed in past studies

4 main differences: ET0 vs ETc

• Crop height– influences the aerodynamic resistance term in the FAO Penman-

Monteith equation – turbulent transfer of vapour from crop into atmosphere.

• Albedo (reflectance) of the crop-soil surface – affected by the fraction of ground covered by vegetation – by soil surface wetness. – influences net radiation of surface, Rn, – primary source of energy for evaporation process.

• Canopy resistance of crop to vapour transfer – affected by leaf area index (number of stomata), leaf age &

condition, & degree of stomatal control. – influences surface resistance.

• Evaporation from soil, especially exposed soil.

Kc predicts ETc under standard

conditions• No limitations are placed on

– crop growth; – evapotranspiration due to water shortage; – crop density; – disease, weed, insect or osmotic potentials.

• ETc predicted by Kc can be adjusted if necessary to non-standard conditions, – where any environmental condition or characteristic is

known to have an impact on or to limit ETc. • Factors for correcting ETc to an adjusted value are

described in Allen et al. (1998) and section 10.6.

FACTORS DETERMINING Kc

1. CROP TYPE• Differences in albedo, crop height, aerodynamic properties, & leaf

and stomata properties, • =>ET from full grown, well-watered crops differs from ET0. • Crop characteristics influence on Kc:

– Taller: Kc up– Shorter: Kc down– Leafier: Kc up– Stomatal control: Kc down– Closer spacing: Kc up

• Crops that grow taller with high roughness of full grown crops – => have Kc factors larger than 1. – 15-20% greater for some tall crops such as maize, sorghum or sugar

cane

FACTORS DETERMINING Kc

• 1. CROP TYPE cont

FACTORS DETERMINING Kc

2. CLIMATE• More arid climates & conditions of greater wind speed

=> Kc up.

• More humid climates and conditions of lower wind speed => Kc down.

• Impact of climate on Kc for full grown crops upper bounds = extremely arid and windy conditionslower bounds = very humid & calm weather conditions.

• Ranges of Kc as climate and weather conditions change small for short crops BUT large for tall crops.

FACTORS DETERMINING Kc

• 2. CLIMATE

FACTORS DETERMINING Kc

3. SOIL EVAPORATION• ET= E + T• Soil evaporation & crop transpiration in field crops

integrated in Kc

• After rainfall or irrigation, – effect of evaporation is dominant – when crop is small & little shade on ground

• For low-cover conditions, – Kc coefficient a/c by frequency of soil surface wetting.

• Where soil is wet for most of time from irrigation or rain, – Kc ≥ 1.

• Where soil surface is dry, evaporation is restricted and – Kc small as low as 0.1

FACTORS DETERMINING Kc

4. CROP GROWTH STAGES• As crop develops,

– ground cover, crop height & leaf area change.

• Due to differences in ET in various growth stages – Kc vary over growing period.

• Growing period divided - 4 distinct growth stages: – initial– crop development – mid-season – late season

MAIZE GROWTH STAGES

PROCEDURE TO GET CROP COEFFICIENT Kc

• ID crop growth stages

• determine lengths of each stage

• select Kc coefficients

• adjust Kc values for frequency of wetting or climatic conditions

• construct crop coefficient curve

• calculate ETc = ET0 x Kc

DEFENITION OF GROWTH STAGES

• Initial stage: sowing to 10% foliage cover

• Development stage: 10% to 80% foliage cover

• Mid-season stage: 80% foliage cover to first signs of senescence

• Late season: First signs of senescence to physiologically inactive

CROP ROWTH STAGES

LENGTH OF GROWTH STAGES

• development stage a/c weather/climate• specific temperature & growth & climate,

latitude, elevation, planting date & crop variety

• mid-season stage a/c genotype & phenology & flowering, seed development, ripening & senescence

• late season a/c frost or harvested fresh = sharp cutoff

Kc ini (initial stage)

• table for planning

• real values a/c • time interval between irrigation events

• evaporation power of atmosphere

• magnitude of wetting even – surface area

• use Kc adjusted

• a/c climate & wind graphs & crop height

Kc dev (development stage)

• From 10% cover to full cover by leaves & closed between rows;

• For grain crops use flowering time

• Leaf Area Index LAI = 3

• If more shadow – lower Es

• Kc = 0,5 if 25-40 % cover

• Kc = 0,7 if 40–60 % cover

Kc mid (mid-season stage)

• a/c climate RHmin

– if humid Kc

– if dry Kc

• frequency of wetting– but smaller effect after full canopy

Kc end (late season)

• fresh harvest irrigate to end / last day

• Esoil high

• mature dry harvest dryout to end

• Esoil low

• arid, high wind high Kc end

• humid, lower wind lower Kc end

GRAPHICAL DETERMINATION of Kc

CONSTRUCT Kc CURVE

Divide growing period into 4 periods– determine lengths of each period– Identify Kc ini; Kc mid; Kc end

Adjust Kc – a/c climate– a/c wetting frequency

Construct curve– connect straight lines– Kc ini horizontal– Kc mid horizontal– join Kc ini & Kc mid diagonal lines– Kc mid to Kc end diagonal lines

CONSTRUCT Kc CURVE

• Forage crops have Kc end after each cut or grazing

• Fruit trees need wind & RH adjustment

• - deciduous Kc end = time of leaf drop

• Determine Kc from graph …………

• Can use any of following:-

– daily, 10 days, monthly values

Estimating irrigation requirements

Soil water balance

I + P - RO - DP - ET S = 0

Where: I=irrigationP=precipitationRO=runoffDP=drainageET = evapotranspirationS = change in soil water content.

• I & P & RO can be measured• D is difficult to measure, usually 0 S is difference between begin & end soil water content• ET can be calculated as difference:- ET = I + P - RO - DP S

Soil water balance

Water available in the soil profile

• TAW = total available water [mm m-1 soil depth]Field capacity – wilting point [mm m-1 soil depth]

• RAW = readily available water [mm (rooting depth)-1]f.rd.(Field capacity – wilting point)where:

f = allowed depletionrd = rooting depth

• Allowed depletion:– “Rule-of-thumb” = 50%– Actual: varies from crop to crop

varies according to crop growth stagevaries according to atmospheric demand

• Rooting depth expressed in m

Typical soil water capacities

Soil FC WP TAW

Clay 360 230 140

Silt-clay-loam 340 210 130

Loam 250 120 130

Sandy loam 230 110 120

Loamy sand 150 70 80

Sand 120 80 40

How much? Amount?

• must use rainfall• use soil water balance

– I + P - RO - DP - ET S = 0– Where I=irrigation, P=rain, RO=runoff, DP=drainage, ET =

evapotranspiration, S = change in soil water content.

• use weather data to calculate ET– use sum of daily values of ET since last I – use FAO Penman-Monteith equations to calculate ET– need temperature (dry bulb & wet bulb), wind, radiation data– Can use crop factor to reduce potential evaporation for crop

condition (age, stress)

• availability of water from dam or river NB

Calculating daily irrigation water requirement

• ET0 = 6.5 mm

• Crop growth stage: Midseason

• Crop Kc = 1.15

• ETc = 6.5 * 1.15 = 7.5 mm

Calculating readily available water

• Soil = Loam• TAW = 130 mm m-1

• ET0 = 6.5 mm• Crop growth stage: Midseason• Crop Kc = 1.15• Crop rooting depth = 1.1 m• Allowed extraction = 50%• RAW = 130 * 1.1 * 0.5 = 71.5 mm

Calculating time to next irrigation

• Soil RAW = 71.5 mm

• Kc = 7.5 mm

• Days before next irrigation =

71.5 / 7.5 = 9.5 days

• We therefore plan to irrigate in 9 days time and then add 9 * 7.5 = 67.5 mm water

Planning irrigation water requirement

On a day by day basis during expected crop growth:• Do water balance calculation

– Rainfall included as per historic data– ETc as demonstrated– Water extraction as demonstrated– Plan irrigation amount when soil water content reaches end of

RAW level– Total irrigation amounts for months and for season

• Total for season is the estimated irrigation requirement

Managing irrigation water

Irrigation water management

• Measure, measure, measure …..• Use irrigation requirement estimate as basis • Measure irrigation water application• Measure rainfall• Measure soil water content• Keep track of water supply at source• Adapt irrigation management plan if necessary

Indicators as when to irrigate

• soil water measurements– use probes, neutron moisture meters, tensiometers, gypsum

blocks• plant water status measurements on weekly basis

– leaf water potential– leaf temperature - infra-red– reduction in transpiration

• If not enough water available, change irrigation strategy to:– Irrigate only during critical stages - e.g. flowering, tillering– Apply deficit irrigation – somewhat less than optimum crop

requirement, increasing water productivity but reducing yield– Economic analysis: full irrigation area at less than maximum

yield vs. smaller area at maximum yield.

Water use planning:using SAPWAT3

SAPWAT3 available from the Water Research Commission:

www.wrc.org.za