Salinity assessment of irrigation water by WatSuit
The WatSuit model
Managing salinity in irrigated agricultural land is a major challenge for sustainable production. To facilitate the evaluation of irrigation water suitability, a computer model, Watsuit, was developed from early 1970s through 1990s by Dr. Jim Oster at University of California-Riverside and Dr. James Rhoades at the US George Brown Salinity Laboratory. Recently the model has been converted to a user-friendly, Website model at University of California, Riverside.
The computer model Watsuit is a steady-state model and assumes a particular leaching fraction remains constant over time. Consequently cation exchange reactions can be and are assumed to be at equilibrium: there is no cation exchange in the model.
Watsuit calculates the concentrations of the major cations (Na+, Ca+, Mg+, and K+) and anions (Cl-, HCO3-, CO32-, and SO42-) of soil water based on the composition of irrigation water and various management practices including: leaching fraction and amendment additions to the water. The model does not use the depth (or volume) of applied water as an input, rather it uses leaching fraction. Because the depth or volume of applied water is not used, the concentrations of ions as well as the amount of precipitated or dissolved salt are expressed as concentrations.
The relative water uptake is assumed to be 40, 30, 20 and 10 % of the total for first (upper), second, third and fourth (bottom) quarters of the rootzone, respectively. The depth of the rootzone is assumed to be constant, but it is not defined.
The user can define the total depth of the rootzone and assume the depth of each quarter of the rootzone is ¼ of the total depth. Or the user can define the depth of each quarter of the rootzone based on the depths at which 40 % (bottom of the first quarter), 70 % (bottom of the second quarter), 90 % (bottom of the third quarter), and 100 % (bottom of the rootzone) of the water uptake has occurred.
The model calculates pH assuming a partial pressure of CO2 in that increases with depth through the soil. At the surface, the PCO2 is set at 0.07 kPa and 0.5, 1.5, 2.3, and 3.0 kPa going from the upper quarter of the rootzone to the last quarter.