Evaluation of irrigation water quality for a spatially variable field

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Abstract
A model to predict the optimal salt concentration of irrigation water when various sources of water are available is described and illustrated for citrus. The model employs a stochastic model for soil water and solute movement and a linear programing economic model for best solutions over an extended time period. The model was developed for a Mediterranean‐type climate where an irrigation (salt accumulation) season is followed by a rainfall (salt leaching) period. Soil water flow generated by an irrigation or rainfall is assumed to be steady and vertical. Transient solute transport is assumed to obey the diffusion‐convection equation. The probability of any given average root zone salinity after any cycle of irrigation or rain is calculated for any combination of applied water salinity and initial soil salinity using a probability density function for the saturated hydraulic conductivity. The expectation of the total income from a crop yield having a known response function to soil salinity is maximized by the model subject to a given probability of the average root zone salinity exceeding a prespecified critical concentration. The cost of water is assumed to decrease as the salinity of irrigation water increases. Optimal water management depending on the salt concentration of the irrigation water over 6 years for citrus is illustrated in terms of minimum cost, maximum income, optimum salt concentration of the irrigation water, spatial distribution of average root zone salinity, and the probability of exceeding a prescribed critical soil salinity concentration.