Using Coupled Hydrologic and Agro-economic Models to Evaluate the Impact of Agricultural Activity on Streamflows

Abstract:

Irrigation substantially alters the timing and magnitude of surface water flows, and continued agricultural intensification to keep up with demand means perpetual stress on surface water resources. A critical challenge is to manage irrigation in a way that balances ecosystem health with sustaining agricultural economies. Coupled hydrologic-agroeconomic models are promising tools for meeting this challenge: the models can quantify 1) how water withdrawal for irrigation impacts streamflows, 2) how these impacts propagate through a surface water system, 3) how the amount of water available for irrigation changes the allocation of resources (e.g. land, water) to available crops, and 4) the impact of water availability on agricultural economies. However, these models can be very data intensive, which limits their applicability. We present a parsimonious coupled hydrologic-agroeconomic model that uses the Positive Mathematical Programming (PMP) method, extensively used in agricultural resource economics, and calibrates to data on allotment of agricultural inputs, available from sources such as the USDA's National Agricultural Statistics Service. PMP assumes that farmers allocate resources to maximize net revenues, justifying the use of optimality conditions to constrain the parameters of the agroeconomic model. We improve the standard PMP model by 1) having the calibrated model reproduce not only the observed input allotment but also the observed yield, and 2) using the ensemble Kalman filter equations to solve the mathematical programming problem recursively, which permits refinement of the model calibration as new observations become available. We demonstrate the proposed agroeconomic model by coupling it to HEC-HMS, a hydrologic model capable of simulating regional natural and man-made water distribution networks, to investigate the sensitivity of streamflows to the allocation of agricultural inputs (land and water) in response to changes in climatic and economic conditions.