SOIL-WATER MODEL FOR EVALUATING WATER DELIVERY FLEXIBILITY

Performance of farm irrigation systems may be limited by water delivery system flexibility. A soil water storage simulation model (SWSSM) was developed to calculate daily soil water storage and corresponding evapotranspiration and deep percolation as a function of water delivery flexibility for a vineyard in California. Uniformity was nearly constant for a range of irrigation time and furrow inflow rate, thus, delivery flow rate and duration were coalesced into delivery depth. Evapotranspiration and deep percolation were simulated for rotation, modified-frequency, modified-amount, and demand schedules. Growers follow a demand schedule during critical growth stages and thereafter a modified-amount schedule with a choice of fixed frequencies. Converting from the actual rotation schedule to the suggested schedule increased seasonal relative evapotranspiration from 0.73 to 0.84 and decreased delivered depth from 1.16 to 0.75 m. Adjustment for extremes in weather is not possible. A suggested water allocation policy provides growers with an incentive to optimize scheduling, but it requires reliability in timing and accuracy in amount delivered.