Irrigation systems are designed to deliver water to crops, but their efficacy varies widely due to operational decisions, weather variability, and water availability. The operation of an irrigation system is studied in this dissertation to determine irrigation performance and potential for improvement.Satellite remote-sensing was used to determine inter-annual variability in crop evapotranspiration and link it with weather patterns and operational decisions. A decade was studied to include several dry, wet and average years of snowfall. It was found that the irrigation district has the capacity to buffer a dry year, but crop evapotranspiration patterns indicated that the buffer capacity of the irrigation district is limited in a second dry year. Studying the current operations of an irrigation system also requires an analysis of the spatial variability within the system to identify potential areas for improvement. Achieving such information is challenging due to the spatial heterogeneity between farm fields. The Ador irrigation system simulation model is used in this study with satellite remote sensing data, which were combined in the calibration and validation process to ease the re-adjustment of management parameters. This approach provides a cost-effective and innovative method for model simulation when field observations are limited. Alternative water delivery scenarios were simulated with the Ador irrigation system simulation model to quantify changes in the water balance, irrigation performance, and water productivity. Results for implementing a minimal irrigation time indicated that irrigation events occurred with a higher frequency and reduced crop water stress. Water productivity for the irrigation district increased substantially in this scenario, whilst district water savings were achieved by diverting less irrigation water. Advantages are only achieved if farmers collectively make the decision to change. A water accounting analysis is required to examine if water savings are achieved at basin scale. There is a potential for the rebound effect to occur, which suggests that an increase of water efficiency causes the increase of water consumption. Simulation results indicated that if the efficiency is increased through improvements of the water delivery, the water consumption increased. Water savings achieved by reducing irrigation diversions did not compensate for the decrease in drainage that downstream users depend on.

The Sevier River Basin situated in south central Utah is characterized by its semiarid climate, snowmelt-driven runoff, and high dependency on agricultural economy. High evapotranspiration and low precipitation make agricultural production challenging, but naturally stored water in the snowpack in the mountains alleviates water stresses during high water demand seasons. The snowmelt-driven river flow along the main channel is highly exploited for irrigation for farms near the Sevier River. Reservoir operations and river diversions result in heavily regulated flows from the upper to the lower basins. The return flows of over-irrigated water in the upper basin increase salinity of surface water. Long-term applications of salinity water in agriculture eventually produce high soil salinity in the agricultural areas near Delta in the lower basin, which deteriorated farmers’ crop productivity. Farmers cropping near Delta struggle with both water and salinity stresses. Indeed, crop prices and yields are always their concerns. For them, efficient water management can be achieved with consideration of hydrologic, agronomic, and economic aspects of water resources. The overall goal of this research was to develop a decision supporting framework for efficient water and land allocations that considered hydrologic processes, crop response to water in salinity-affected farms, and farmers’ profit and financial risk. This research introduces a methodology for predicting water availability in a given cropping year from the snowpack in the mountains, and agronomic simulations with satellite images follow for quantifying crop response to water. The hydrologic predictions and the agronomic simulations are finally incorporated into an economic analysis that provides efficient water and land allocations with multiple crop selections. In a rural river basin, data limitation is a common concern for water resources engineers; thus simple but robust methodologies are proposed for hydrologic prediction. In the same context, satellite images are used for the estimation of crop yields in individual farms near Delta with no prior crop experimental plots. Historical records of crop prices are used for the economic analysis. The methodologies developed in this research provide a comprehensive decision analysis framework for efficient water management where water is scare and available from snowmelt only, the economy depends on agriculture only, and salinity is present in both soil and water due to long-term irrigation. The case study is for the agricultural area near Delta in the Sevier River Basin, but its applicability is not limited and is flexibly applicable to other agricultural regions.

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