Hydro-economic modeling of managed aquifer recharge for an agricultural landscape

Abstract:

The Mississippi Embayment is the second most depleted aquifer in the USA. The greatest losses in groundwater storage occurred in the uppermost aquifer of the Embayment, the Alluvial aquifer, especially in Eastern Arkansas. Previous studies have shown that the adoption of efficient irrigation methods has not been effective in reducing groundwater overdraft. One potential solution to conserve groundwater is the conjunctive use of surface and groundwater through managed aquifer recharge (MAR). However, little information is available about the economics of MAR considering the complexity of groundwater flow and the variety of crops grown in the Eastern Arkansas region. In this study, we develop a coupled hydro-economic model for Eastern Arkansas that integrates an economic model with a MODFLOW groundwater flow model to evaluate decadal changes in cropping systems, aquifer storage, and farm net return. The finite-difference grid of the groundwater flow model has 3000 cells with one-square-mile each and covers ten primary hydrogeologic units with 13 groundwater layers. The economic model is formulated as a mathematical optimization problem. The objective function employed in the hydro-economic models is the sum of net economic benefits from crop production minus the cost of irrigation, MAR, and maintaining on-farm reservoirs, which vary across hydro-geologic aquifer characteristics and agronomic and economic conditions. We automate the execution of the economic and hydrologic models and the data exchange process at each time step with a wrapper program written in Python. To evaluate the economics of MAR, we run the model for 60 years with different costs of MAR water. Our analysis shows that MAR alone unlikely eliminates the groundwater depletion in the region, even with an optimistically low cost of MAR water ($20/acre-foot). Still, MAR leads to higher farm net return when compared with the no MAR scenario. The results further show that the groundwater heads decline, on average, 0.15 and 0.20 feet per year when MAR cost is $20 and $60/acre-foot, respectively. With low MAR cost ($20/acre-foot), MAR increases farm net return by more than 2% annually compared to no MAR scenario. Interestingly, the results also show that optimal MAR does not always occur in cells with thin saturated thickness, implying that active recharge of water into aquifers that are more depleted does not necessarily lead to greater benefits to society.