Simulating Spatially Distributed Water and Salt Balances

Irrigation districts are made up of multiple units that are hydrologically connected. If the irrigation units are arranged in parallel, an improvement of the on-field irrigation consumptive use coefficient (ICUC) has more impact on the district ICUC than if the irrigation units are arranged in series. But water quality degrades with the number of reuses. Knowing the topological relations of the irrigation units, the water use in an irrigation district may be analyzed at high and low spatial resolution. A site-specific analysis was conducted on a complex irrigation district (Tulelake Irrigation District) located in a valuable ecosystem in Northern California. Simulated applied water and drainage were greater for areas having high water-consuming crops. The estimated concentration of salts was higher in the center and southern areas. Concentration was higher for soils high in organic matter and for areas where water was reused several times. For comparison purposes, even a complex irrigation district may be reduced to an equivalent number of command areas arranged perfectly in series.