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Drought Planning for Vegetable Production 1
Introduction 2
Under drought conditions when water may be in short supply, growers often need to 3
estimate the potential water use of their crops to determine how much land can be irrigated. 4
Additionally, these calculations may be needed when switching to a different crop type under a 5
limited water supply. It is also necessary to estimate crop water requirements when buying or 6
leasing land to determine if a potential water supply is sufficient to irrigate the entire farm. The 7
following sections detail the calculations involved in estimating crop water requirements and 8
determining how much land can be irrigated under limited water supplies. Example calculations 9
are provided as well as a discussion of the key considerations in developing accurate estimates. 10
Defining crop water needs 11
The volume of irrigation water needed to produce a crop is the difference between crop 12
water needs and contributions of water from non-irrigation sources, such as precipitation from 13
rain and fog. Other non-irrigation sources of water include moisture contributed by shallow 14
water tables, and moisture stored in the soil profile. 15
Water is needed for crop production both pre-season (before planting) and in-season. 16
Water may be needed pre-season for preparing soil for planting and leaching salts, and during the 17
season for evapotranspiration, controlling salinity, and to compensate for inefficiencies in the 18
irrigation application. In some cases water is needed during the season for frost protection or for 19
increasing humidity and lowering the air temperature during hot periods. Each component of 20
in-season and pre-season crop water needs must be estimated to determine the overall water 21
needs of a vegetable crop. 22
Estimating in-season water needs 23
Crop evapotranspiration 24
Evapotranspiration (ET) is the water that crops lose by evaporation from the soil and wet 25
plant surfaces and through transpiration. Plants transpire water through stomata, small openings 26
on leaves, where liquid water transforms to vapor and is lost into the air. The daily ET rate is 27
usually expressed in inches or millimeters per day and depends on weather and crop factors. 28
Potential ET is the amount water a crop uses for evapotranspiration when it is fully 29
irrigated. Yields of vegetable crop are usually maximized when moisture is sufficient to meet the 30
potential ET rate throughout the season. A crop is under water stress when the supply of soil 31
moisture is insufficient to meet potential ET requirements. Short periods of water stress may 32
reduce the growth rate of crops. Prolonged periods of water stress will cause the pores on leaves 33
that transpire moisture, called stomata, to close to prevent water losses. The closure of stomata 34
reduces the efficiency of the crop to uptake and incorporate carbon dioxide into sugars. 35
Consequently, prolonged water stress will cause yield losses, especially if water stress occurs 36
during critical periods of crop development such as flowering or fruit development. ET of a crop 37
under water stress will be less than a crop watered to meet potential ET requirements. 38
Potential crop ET will vary depending on where and when a crop is grown, and the crop 39
type. Reference ET values are available for most agricultural regions of the state from the 40
California Irrigation Management and Information System (CIMIS) managed by the California 41
Department of Water Resources (CDWR) through their website (www.cimis.water.ca.gov). 42
Reference ET values are from weather stations located on a reference crop (Figure 1), usually 43
grass or alfalfa, with a consistent and full cover of vegetation. Historical daily reference ET 44
values are available for each region, as well as historical monthly averages of ET. The 45
reference ET values must be converted using crop coefficients to estimate ET for a different 46
crop. ANR publication 3396 explains how to estimate crop ET using crop coefficients. In 47
addition, a number of web-based programs can help calculate crop ET using reference ET values 48
and crop coefficients. WaterRight (www.wateright.org), Irrigation Scheduler Mobile 49
(weather.wsu.edu), and CropManage (cropmanage.ucanr.edu) are examples of online 50
applications that can be used to estimate ET of vegetable crops using CIMIS reference ET data. 51
Table 1 summarizes crop ET values for a range of vegetables produced on the Central Coast of 52
California. These values can be used as approximations for ET requirements for these 53
commodities for the Central Coast region. 54
Table 1. Estimated ET of selected vegetable crops grown on the Central Coast. 55
Region Crop
Irrigation
Method Season
ET
requirement
(inches)
Central Coast (coastal valleys)
Broccoli Sprinkler March - August 6
Broccoli Sprinkler May - September 7
Broccoli Sprinkler September - March 6
Brussels sprouts Sprinkler July - December 14
Cauliflower Sprinkler March - July 7
Cauliflower Sprinkler August - December 6
Cabbage Sprinkler August - October 10
Cabbage Sprinkler April - July 11
Celery Sprinkler/Drip May - September 8
Lettuce (iceberg) Sprinkler/Drip February - May 5
Lettuce (iceberg) Sprinkler/Drip April - July 7
Lettuce (iceberg) Sprinkler/Drip June - August 6
Central Coast (interior valleys)
Bok Choy (greenhouse) Sprinkler July-September 4
Broccoli Sprinkler March - August 13
Broccoli Sprinkler May - September 12
Broccoli Sprinkler September - March 6
Cabbage Sprinkler April - August 15
Cauliflower Sprinkler March -July 12
Cauliflower Sprinkler November - April 12
Celery Drip May - August 12
Lettuce (iceberg) Sprinkler/Drip April - July 10
Lettuce (iceberg) Sprinkler/Drip June - August 8
Lettuce (iceberg) Sprinkler June - August 9
Peppers Drip May - September 14
Spinach (Baby) Sprinkler May - September 3
Tomatoes (Fresh Market) Drip June -September 13
Tomatoes (Processing) Sprinkler/Drip May - September 14
56
Application efficiency 57
Application efficiency (also known as irrigation efficiency) needs to be factored into the 58
irrigation requirement of a crop. Application efficiency is defined as the fraction of applied 59
water that remains in the root zone where it can be used by the crop. A portion of applied water 60
may be lost to drainage or run-off. Both the distribution uniformity of an irrigation system and 61
scheduling of irrigations can affect application efficiency. Assuming that the irrigation schedule 62
is optimized by good management, distribution uniformity (DU) may be considered a close 63
approximation of application efficiency. A high DU means that an irrigation system applies 64
water uniformly in a field. Most well-designed and operated drip systems have a DU greater 65
than 85%. Well maintained and operated sprinkler systems have DU values ranging between 66
70% and 80%. Growers often allow for a low application efficiency during the establishment 67
of vegetable crops to assure that they attain a uniform stand of plants. Hence it may be 68
advisable to separate the application efficiency for establishment and post-establishment water. 69
Salt management 70
Leaching needed to control salinity in the root zone increases the water needs of a crop. 71
ANR publication 8550 entitled βManaging salts by leachingβ describes how to estimate leaching 72
requirements (LR) of vegetable crops for specific water and soil salinity conditions. Leaching 73
can be accomplished both pre-season and during the cropping season depending on the water 74
availability and water quality. 75
The following equation can be used to estimate applied water needed to meet in-season 76
crop water requirements: 77
π΄π = πΈππ Γ 100%/π·π
1 βπΏπ
100%
78
where AW is applied water in inches, ETc is crop evapotranspiration in inches, DU is percent 79
distribution uniformity (%), and LR is percent leaching requirement. 80
81
Example: How much water will be needed for the in-season water requirements of summer 82
lettuce irrigated with sprinklers and grown in the interior valleys of the Central Coast if the 83
average DU is 70% and the leaching fraction needed is 10%? 84
According to Table 1, sprinkler-irrigated lettuce planted in June needs 9 inches of water 85
for ET. Using the above equation, 14.3 inches of applied water is needed to grow a lettuce crop. 86
π΄π = 9 πππβππ Γ 100%/70%
1 β10%
100%
87
π΄π = 14.3 πππβππ 88
Crop establishment water 89
Vegetables often need more water than crop ET for germinating seeded crops or for 90
overcoming transplanting shock (Figure 2). The amount of establishment water will depend on 91
the existing moisture in the soil, length of time for germination or transplant establishment, 92
application efficiency, and weather conditions. In practice, most growers that plant or establish 93
vegetable transplants in moist soil will apply at least twice the reference ET Γ number of days for 94
germination or transplant establishment. 95
Example: What is minimum applied water (AW) needed for germinating a lettuce crop if 96
average reference ET = 0.17 inches per day and seed germination = 7 days? 97
AW for germination = 2 Γ0.17 inches/day Γ 7 days = 2.4 inches 98
Frost protection, humidity, and temperature control 99
Irrigation is sometimes needed to influence the micro-climate of a crop for short periods. 100
Irrigation water applied with sprinklers can protect a crop from frost damage that may occur 101
during cold nights, increase humidity when hot dry winds blow, or decrease damage from 102
excessively hot conditions. It may be difficult to anticipate the amount of water needed for 103
affecting the micro-climate of a crop, but past experience and weather records may provide a 104
rough guide. 105
Estimating pre-season water needs 106
Water applied before planting needs to be factored into the crop water requirements. An 107
application of pre-season water may be needed to bring the soil to field capacity before planting 108
or tillage operations. If pre-season rainfall was insufficient, additional water may be needed to 109
leach salts that have built up in the soil. Often pre-season water can be minimized by evaluating 110
soil water status and determining the amount of water needed to bring the soil to saturation. 111
The required amount of pre-season water can be estimated by comparing the current water 112
content of the soil with the water content at field capacity. Table 2 estimates the water content of 113
soils of a range of textures at field capacity (~30 cbars). 114
Table 2. Texture effects on water holding capacity of soils. 115
116
Soil Texture
Field Capacity
(30 cbar)
Wilting Point
(1500 cbar)
Available
Moisture
----- inches of water per inch of soil depth-----
Sand 0.10 0.04 0.06
Loamy Sand 0.16 0.07 0.09
Sandy Loam 0.21 0.09 0.12
Loamy Sand 0.27 0.12 0.15
Silt Loam 0.30 0.15 0.15
Sandy Clay Loam 0.29 0.18 0.11
Sandy Clay 0.28 0.15 0.13
Clay Loam 0.32 0.18 0.13
Silty Clay Loam 0.36 0.20 0.16
Silty Clay 0.40 0.20 0.20
Clay 0.40 0.22 0.18
117
118
Example: If the water content of a clay loam soil averages 25% to a 3 foot depth, how much 119
water will be needed to bring the soil profile to field capacity? 120
Table 2 shows that the water content of a clay loam soil at field capacity is 32% or 0.32 inches 121
per inch of soil depth. The amount of water needed to bring the soil to field capacity equals: 122
(0.32 β 0.25) Γ 3 ft Γ12 inches/ft = 2.5 inches 123
Estimating non-irrigation sources of water 124
Non-irrigation sources of water can satisfy a portion of the crop water needs. These 125
sources include water from rainfall, fog, and shallow water tables. Moisture stored in the soil 126
profile can also provide a portion of the water need to produce a vegetable crop. 127
Soil moisture contribution to crop water requirements 128
Even during a drought, a significant portion of the moisture contributing to crop ET may 129
be stored in the soil after pre-season rains or after pre-season irrigations. Soil moisture 130
contributing to crop water use is the difference in soil water content between planting and 131
harvest. 132
Soil moisture available for a crop can be estimated from the water-holding characteristics 133
of a soil. Soil water-holding characteristics describe the amount of water a soil can hold and how 134
tightly water is held at different water contents. Soils with a high percentage of clay and silt 135
sized particles (clay, clay loam) hold more water than soils with a high percentage of sand (sand, 136
sandy loam). Moisture in fine textured soils, such as clay loam soils, is held tighter than in 137
coarse textured soils such as sandy loam soils. A portion of the water held in the pores of a soil 138
is available to a crop and the remainder is unavailable because it is held too tightly for crop roots 139
to extract. Generally, 15 bars is used as a standard tension at which most plants cannot extract 140
water from soil pores. Available water is the difference between the water content of the soil at 141
field capacity (water content of a soil after it is saturated and excess water drains) and the water 142
content at wilting point. Table 2 provides estimates of available water for soils of a range of 143
textures. Allowable depletion is the percentage of available soil water that can be taken up by a 144
crop before the soil water content is so low that production and yield are adversely affected. 145
Table 3 summarizes allowable depletions for common vegetable crops. For the purpose of 146
estimating the change in soil moisture storage over a crop cycle, the allowable depletion should 147
equal the soil moisture content at harvest. Rooting depth at harvest (Figure 3) also needs to be 148
considered in the estimate because the greater the root depth the more soil moisture that the crop 149
can extract. Table 4 summarizes common rooting depths of vegetable crops. 150
151
Table 3. Allowable depletion of available soil moisture at crop maturity. 152
Crop
Allowable depletion
of available soil
moisture (%)1 Crop
Allowable depletion
of available soil
moisture (%)1
Artichoke 45 Lettuce (leaf) 25
Asparagus 40 Lettuce (baby) 25
Bean (green) 30 Melons (mixed) 60
Bean (snap) 30 Okra 65
Beet (table) 50 Onion (dry) 40
Broccoli 40 Onion (green) 25
Brussels sprout 50 Parsnip 40
Cabbage 35 Pea 30
Cantaloupe 40 Pepper (Bell) 25
Carrot 35 Potato 35
Cauliflower 40 Potato (sweet) 65
Celery 25 Pumpkin 50
Chard 30 Radish 35
Cilantro (Coriander) 30 Spinach 35
Corn (sweet) 30 Spinach (baby and teen) 25
Cucumber 50 Squash (summer) 35
Eggplant 30 Squash (winter) 60
Garlic 50 Tomato (fresh) 35
Kale 50 Tomato (processing) 60
Leek 35 Turnip (white) 35
Lettuce (iceberg) 30 Watermelon 40
Lettuce (romaine) 30
1. Sources: Chapt. 15. Vegetable Irrigation, Irrigation of Agricultural Crops, ASA Monograph # 153
30., 2nd Ed. Hanson et. al. 2004. Scheduling Irrigations. ANR pub 3396. M. Cahn (personal 154
communication with growers, consultants, and farm advisors) 155
156
Table 4. Range of rooting depths of common vegetables at maturity. 157
Crop
Rooting Depth
at Maturity1 (ft) Crop
Rooting Depth
at Maturity1 (ft)
Artichoke 2.0 - 3.0 Lettuce (leaf) 1.0 - 1.5
Asparagus 2.5 - 3.5 Lettuce (baby) 1.0 - 1.5
Bean (green) 1.5 - 2.0 Melons (mixed) 2.0 - 4.0
Bean (snap) 1.5 - 2.0 Okra 2.0 - 3.0
Beet (table) 1.0 - 1.5 Onion (dry) 1.5 - 2.0
Broccoli 2.0 - 3.0 Onion (green) 1.0 - 1.5
Brussels sprout 2.0 - 3.0 Parsnip 2.0 - 3.0
Cabbage 2.0 - 3.0 Pea 1.5 - 2.0
Cantaloupe 2.0 - 4.0 Pepper (Bell) 2.0 - 3.0
Carrot 1.5 - 2.0 Potato 1.5 - 2.0
Cauliflower 2.0 - 3.0 Potato (sweet) 2.0 - 3.0
Celery 1.5 - 2.0 Pumpkin 3.0 - 4.0
Chard 2.0 - 3.0 Radish 1.0 - 1.5
Cilantro (Coriander) 1.5 - 2.0 Spinach 1.5 - 2.0
Corn (sweet) 3.0 - 4.0 Spinach (baby and teen) 1.0 - 1.5
Cucumber 1.5 - 2.0 Squash (summer) 2.0 - 3.0
Eggplant 2.0 - 3.0 Squash (winter) 2.5 - 3.5
Garlic 1.5 - 2.0 Tomato (fresh) 2.0 - 4.0
Kale 2.0 - 3.0 Tomato (processing) 3.0 - 5.0
Leek 1.5 - 2.0 Turnip (white) 1.5 - 2.5
Lettuce (iceberg) 1.5 - 2.0 Watermelon 2.0 - 3.0
Lettuce (romaine) 1.5 - 2.0
1. Source: Chapt 11. Sprinkler Irrigation Section 15, Soil Conservation Service National 158 Engineering Handbook, Chapt. 15. Vegetable Irrigation, Irrigation of Agricultural Crops, ASA 159 Monograph # 30. 2nd Ed. M. Cahn (personal communication with growers, consultants, and farm 160 advisors) 161
162
Example: How much soil moisture is available to a lettuce crop planted on clay loam soil? 163
Available moisture for a clay loam soil is approximately 0.13 inches per inch depth of 164
soil. The allowable depletion of available soil moisture for lettuce is 30%, and roots of a typical 165
lettuce crop reach a depth of 24 inches (2 feet) by maturity. 166
Available soil moisture = 0.13 inches per inch of soil Γ 30%/100% Γ 24 inches 167
= 0.9 inches 168
Precipitation and shallow water tables 169
Other sources of water that can contribute to crop ET include rainfall, fog, and shallow 170
water tables. Rainfall can be significant during the winter, early spring or late fall in some 171
regions of the state but is difficult to predict. Long term rainfall records, available from CIMIS 172
and other agencies, can be used to give a general sense of how much rain is typical, but using a 173
conserve estimate is recommended. Near the coast, fog often contributes precipitation to crops 174
during the summer. Amounts are usually less than a few tenths of an inch per year in most 175
regions of the coast. Water from perched or shallow water tables can move upward into the root 176
zone of a crop. The amount of water contributed from a shallow water table is difficult to 177
assess, but may depend on the soil type, rooting pattern of the crop, and when the water table is 178
present during the crop cycle. If the water table is more than 2 feet below the rooting zone, the 179
contribution of moisture is probably negligible. 180
Determining the overall irrigation water requirement 181
The overall irrigation requirement for a crop based on the above examples can be 182
estimated by summing the pre-season and in-season water needs and subtracting for non-183
irrigation water sources: 184
185
Crop water needs
Pre-season water 1.5
In-season water
Water for establishment 2.4
ETc, Leaching, DU 14.3
Temperature and humidity 0.0
Total 18.2
Non-irrigation water sources
Change in available soil moisture 0.9
Rainfall 1.0
Fog 0.1
Shallow ground water 0.0
Total 2.0
Irrigation water requirement
Crop water needs 18.2
Non-irrigation water sources 2.0
Irrigation water needed 16.2
186
Determine how much water will be available for irrigation 187
In addition to estimating the crop water needs, one needs an estimate of the amount of 188
water available for irrigating during the upcoming season for calculating the acres that can be 189
farmed. Water availability will depend on the source of the water, water rights, and local and 190
state regulations. Water districts that are allocated surface water may not know the final water 191
allotment until late in the spring when most of the precipitation for the season has occurred. In 192
this case, it may be most useful to estimate the land that can be planted for a range of water 193
allocation scenarios (20%, 30%, 40%, 50%, etc. allocations). 194
Growers that rely on ground water for irrigation should conduct a pump test to determine 195
how much water their wells can produce. If aquifer levels have declined during a drought, then 196
wells will likely produce less water. A pump test will determine the flow rate (gallons per 197
minute) that a well can produce at different output pressures. If high pressure sprinklers will be 198
used for irrigating, a pump will have a lower output flow rate than if drip tape, which requires 199
less pressure, will be used. 200
Calculating the total acres that can irrigated 201
The maximum acres that can be irrigated is determined by dividing the total available 202
water by the water requirement of the crop. For example, if a grower is allocated 500 acre-ft of 203
water, then for the above example he can farm 370 acres of lettuce: 204
πΆππππππ ππππ = 500 ππππ β ππ‘ Γ 12 πππβππ /ππ‘
16.2 πππβππ 205
πΆππππππ ππππ = 370 πππππ 206
Allowances for Peak ET 207
In addition to calculating the area that can be irrigated using the total amount of water 208
available for the season, one should also calculate the area that can be irrigated based on the 209
amount of water available per day during the period when the crop has the greatest 210
evapotranspiration requirement. Historical CIMIS ET data can provide maximum values for 211
reference ET during the growing season. The maximum crop coefficient value, which should 212
range between 0.8 and 1.15 depending on the crop, should be used to calculate the peak ET 213
requirement. Bulletin?? 214
Example: How many acres could be cropped if a ranch has 2 wells with a combine 215
output of 1500 gpm, and the peak crop ET is expected to be 0.28 inches/day in the summer? 216
Assuming that the wells can be operated 24 hours per day: 217
πΆππππππ ππππ = 1500 πππ Γ 24 βππ’ππ
0.28 πππβππ Γ 453 218
πΆππππππ ππππ = 284 πππππ 219
where 453 in the above equation is a combined constant for converting gallons to acre inches and 220
hours to minutes. 221
Summary 222
Under drought or when water supplies are limited, estimating the water requirement of 223
crops is necessary to avoid planting more land than can be irrigated. Both pre-season and in-224
season water requirements should be estimated. The major crop water needs in-season are for 225
stand establishment, evapotranspiration, and managing salinity through leaching. The pre-season 226
water needs are mainly for leaching salts that may have built up in the soil from previous crops, 227
and for providing optimal moisture for tillage operations. Non-irrigation sources of water such 228
as precipitation and changes in stored soil moisture can be subtracted from the overall irrigation 229
requirements of the crop. Dividing the total amount of water available for irrigation by the crop 230
water requirement provides an estimate of the total acres that can be productively farmed. 231
Further Reading 232
Cahn, M., and K. Bali. 2016. Drought tip: βManaging salts by leachingβ ANR #8550 pp. 8. 233
Hanson, B., L. Schwankl and A. Fulton 2004. Scheduling irrigations: when and how much 234
water to apply. ANR publication 3396. pp. 202. 235
Lascano, R.J. and R.E. Sojka, 2007. Irrigation of agricultural crops. American Society of 236
Agriculture. Madison, Wisconsin, USA. pp. 664. 237
Hanson, B. 2000. Irrigation pumping plants. ANR publication 3377. pp. 126. 238
239 Figure 1. Weather station used to estimate reference evapotranspiration. Photo credit: M. Cahn. 240
241
242
243
244
245
246
Figure 2. Overhead sprinklers are used to irrigate recently transplanted vegetables. Photo credit: 247 M. Cahn. 248
249
Figure 3. A mature iceberg lettuce crop may have roots below 2 feet of depth. Photo credit: M. 250
Cahn. 251
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