Managing irrigation with reclaimed water: Minimizing negative impacts on soil and groundwater quality

Over-irrigation with reclaimed water may cause crop yield reduction and groundwater quality degradation. Continuous and automatic monitoring strategies are desirable as a means of guiding management schemes to avoid these problems. In this work, an optimal irrigation management scheme known as Receding Horizon Control (RHC) is proposed to balance water reuse and soil/groundwater quality. RHC is a control theory that is proposed here as a method to drive a field site to a desired environmental state with respect to soil and groundwater systems. In this work, RHC is supported using sensor measurements, physically-based state prediction models, and various optimization algorithms. A simulation model including a one- (vertical) dimensional form of the Richards equation coupled to energy and solute transport equations is employed as a state estimator to provide updated soil moisture, temperature, and nitrate data on a regular basis. An optimization algorithm determines the optimal irrigation rate using current state status which continuously maximizes the reclaimed water usage while maintaining water and contaminants (such as nitrate and salinity) in soils at a certain level. Simulated results for soil moisture or nitrate control in the context of a center-pivot irrigation system and a small-scale field test of salinity control demonstrated that the proposed RHC was capable of maintaining specified soil moisture/nitrate/salt levels below threshold values in response to different optimization horizons, optimization algorithms, and prescribed soil depths where the threshold constraint was applied. Sensitivity analysis for solute control (nitrate in this study) demonstrated that the response of solute concentration in soil (control output) to the change of application rate (control variable in this study) is delayed relative to the analogous moisture-only case. This finding suggests that it may be necessary to apply an auxiliary tool in some cases to control a system with respect to solute transport. In conclusion, RHC scheme is a viable strategy for achieving water reuse and agricultural objectives while minimizing negative impacts on environmental quality.