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WATER BALANCE WORK SHEET
This soil water balance spreadsheet program has been developed for Irrigation District personnel as a simple irrigation decision-making support tool to forecast and predict irrigation schedules. This would enhance the computing capacity of irrigation district personnel servicing farms. The tool requires limited weather, crop, and soil data to schedule irrigations over the growing season. Ditch tenders can also use the spreadsheet program to predict farm irrigation requirements.
This spreadsheet scheduling program is based on the soil water balance in the root zone in terms of depletion:
Dr, i = Dr, i-1 - (P-RO)i-li-CRi+(actual)Etc,i+DPi
Dr,i root zone depletion at the end of day i [inch]
Dr,i-1 water content in the root zone at the end of the previous day, i-1 [inch]
Pi precipitation on day i [inch]
ROi runoff from the soil surface on day i [inch]
Ii net irrigation depth on day i that infiltrates the soil [inch]
CRi capillary rise from the groundwater table on day i [inch]
ETc,i actual crop evapotranspiration on day i [inch]
DPi water loss out of the root zone by deep percolation on day i [inch]
Crop evapotranspiration (ETc) is calculated by multiplying the weather-based reference evapotranspiration (ETo) by the crop coefficient (Kc), ETc = ETo * Kc (adjusted for soil moisture stresses, if any). The daily ETo values can be taken from the California Irrigation Management Information System (CIMIS) automated weather stations, which are managed by the California Department of Water Resources (DWR). The daily precipitation data (P) can be taken from the CIMIS stations as well. Net irrigation depth (I) is calculated to satisfy crop evapotranspiration. All the excess water from the root zone due to rain occurs as deep percolation (DP) when the root zone water content exceeds maximum holding capacity of the soil following heavy rain events. No surface runoff (RO) is assumed due to rainfalls, and daily precipitation in amounts less than 20 percent of ETo is ignored as it is normally evaporated. The upward water transfer by capillary rise (CR) from shallow water tables towards the
root zone is difficult to assess and thus ignored in depletion calculations.
In this spreadsheet program, crop ET is adjusted for soil water stress, if any:
ETc actual = ETo * Kc *Ks
Ks = (TAW – Dr)/(TAW – RAW)
where ETc actual is the ET rate that occurs under actual field condition, Ks is the adjustment coefficient for water stress, TAW is the total available water in the root zone and Dr is the root zone depletion.
The main reference used to develop this spreadsheet program is FAO 56 (Allen et al., 1998); the secondary reference is ASCE No. 70 (Jensen and Allen, 2015).
This irrigation scheduling spreadsheet program is very simple and quick to use. You only need reference evapotranspiration and rainfall data from the preferred CIMIS station, and a minimum amount of crop and soil data. References are given in the spreadsheet program to help with the required data. Required inputs for the spreadsheet program include weather data, crop data, and soil data.
The weather inputs include daily reference evapotranspiration (ETo) and daily precipitation. This data will be downloaded from the selected CIMIS station for the selected growing season. Moreover, the weather data can also be selected for a typical dry year, typical wet year, or a typical average year to identify the highs and lows of farm irrigation water requirements.
The following dates are required to construct the crop coefficient curve:
Canopy cover exceeds 10 percent;
Canopy cover exceeds 70 percent;
Beginning of senescence; and
End of season.
The crop coefficient values depend on crop type, stage of growth and environmental conditions, particularly wind and relative humidity. In this irrigation scheduling spreadsheet, crop coefficient curves are constructed by selecting three Kc values and dividing the growing season into four periods: initial, crop development, midseason, and late season.
Initial crop coefficient: average Kc during the initial period;
Maximum crop coefficient: average Kc during midseason period;
End of season crop coefficient: average Kc at the end of late season.
Reference Kc values and lengths of growth stages are provided in the scheduling spreadsheet. Length of growth stages, however, are influenced by crop variety, local conditions, and time of year.
Crop root inputs include initial root depth and maximum root depth. For annual crops, the initial root depth can be taken as the seeding depth plus an additional depth following germination, usually about six inches. For perennial crops, the initial root depth is the same as the maximum root depth for irrigation scheduling purposes.
The soil inputs include various water-related data as follows:
Permanent Wilting Point;
Estimated Soil Water Content at Planting/Leaf Out;
Management-Allowed Depletion, MAD; and
Soil Volume Used to Store Water.
Field capacity and permanent wilting point
Reasonable estimates of field capacity and permanent wilting point can be obtained by soil texture as shown in the following table. Soil available water is the difference in water content between field capacity and permanent wilting point.
Estimated Soil Water Content at Planting/Leaf Out
Soil water content at planting or leaf out must be estimated to initiate the root zone water balance.
Management-Allowed Depletion (MAD)
Only a portion of available soil water is “readily” available to the plant depending on crop and stage of growth, soil type, and evaporative demand. The concept of management-allowed depletion is used in scheduling irrigations to avoid crop water stress, and is defined as the percentage of available soil water that is depleted between irrigations. Values of MAD are typically about 25 to 40 percent for high-value, shallow-rooted crops; 50 percent for deep-rooted crops; and 60 to 65 percent for low-value, deep-rooted crops (Jensen and Allen, 2015).
Allowable Depletion = (MAD/100) * AW * Rz
where AW is available soil water (between field capacity and permanent wilting point) and Rz is effective rooting depth. Another parameter used in irrigation scheduling is the percentage of available water (p) that can be depleted from the root zone before reduction in crop evapotranspiration occurs. This parameter defines ‘readily available water’ (RAW), which is the depth of water that could be depleted between irrigations without suffering plant water stress:
Readily Available Water = (p/100) * AW * Rz
Reference p fractions for various crops are provided in the spreadsheet program.
Soil Volume Used to Store Water
The last required input in the soil section is ‘soil volume used to store water’. Unlike flood and sprinkler irrigation, localized irrigation systems only wet a portion of the root zone, which will affect the frequency of irrigations. This is particularly important in microirrigation with wide drip hose spacing in orchards. A sample calculation is provided in the spreadsheet program.
The outputs of the spreadsheet include irrigation schedules and daily values of crop coefficients, potential and actual crop evapotranspiration, and various soil water data and soil water balance. Sample irrigation schedule and daily values of crop and soil water data are shown below.
Various charts are included in the spreadsheet program showing crop coefficients, soil water storage, crop evapotranspiration, net irrigation, cumulative irrigation, and rainfall during the growing season.
If you have any questions or comments about this spreadsheet program, please contact:
Shawn Ashkan - Agricultural Engineer
Center for Irrigation Technology
California State University
5370 N Chestnut Ave Fresno, CA 93740
FAO 56. 1998. Richard G. Allen, Luis S. Pereira, Dirk Raes, and Martin Smith. FAO Irrigation and Drainage Paper No. 56. Crop Evapotranspiration (guidelines for computing crop water requirements).
ASCE No. 70. 2015. Marvin E. Jensen and Richard G. Allen (Editors). Evaporation, Evapotranspiration, and Irrigation Water Requirements. ASCE Manuals and Reports on Engineering Practice No. 70 (Second Edition).