![]() However, for most soils, some of the excess water is still available to the plant during deep percolation. The new deficit balance is calculated by the checkbook spreadsheet and generally set to zero. If a daily rainfall or irrigation event minus the daily crop water use is greater than the current deficit, consider most of the excess to be lost due to deep percolation below the rooting zone. Table 1: Average daily net application depths for various pumping capacities However, in-field measurements of irrigation water applied is more accurate. You can also estimate the net irrigation amount from Table 1 by knowing the irrigation system's pumping capacity, application efficiency and operating time. To be fairly accurate, rain gauges should have an opening of at least 2 inches and be positioned near the top of the crop canopy in the irrigated field. Read rain gauges within a day after a precipitation event. If you have soil moisture sensing devices in the field, placing a rain gauge at each site would be a good choice. Locate at least two rain gauges within the field to give representative values of the net water received from either rainfall or irrigation. Measuring soil water inputsĪccurately measuring rainfall and irrigation water are essential in estimating a field’s daily soil water deficit. When the soil water deficit exceeds the maximum allowable deficit, then irrigation should be scheduled. SWD(today) =SWD(yesterday) - Rainfall - irrigation + evapotranspiration + water loss (percolation or runoff) The general formula for calculating the daily water balance is expressed in rainfall depth equivalents (in): It forms the basis for making irrigation decisions. With the checkbook spreadsheet, this is calculated automatically. If you are unsure how long to keep your holding register, contact the British Cattle Movement Service ( BCMS).The soil water deficit is the amount of water needed to fill the soil profile to field capacity.
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