Presentations 2016

Incorporating a dynamic irrigation demand module into an integrated groundwater/surface water model to assess drought sustainability

Earthfx Inc.

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Understanding the cumulative effects of pumping, irrigation, drought and groundwater/surface water interaction is central to the agricultural water management problem. Assessing all of these complex processes within a single modelling framework is optimal, but difficult because of the dynamic interaction between climate, soil moisture and irrigation patterns. The USGS has developed, in parallel, two important model codes to study these processes. The USGS GSFLOW code combines the PRMS hydrologic model with the MODFLOW-NWT groundwater model into a fully integrated model that is ideal for studying groundwater and surface water interaction under drought and climate change conditions. Meanwhile, the USGS MODFLOW-OWHM model includes the Farm Process module that represents dynamic farm pumping, irrigation and return flows as a function of soil water demand. Earthfx has created a unified tool to address this challenge by modifying the GSFLOW code to incorporate the approaches and features of the Farm Process module. This evolution of the integrated GSFLOW model can simulate daily irrigation water use based on dynamic soil moisture conditions. Water is applied to the farms by adding the pumped volume to either daily precipitation or throughflow (after interception) based on irrigation method. Return flows are estimated directly by PRMS as overland runoff or interflow and routed to streams. Additional modifications were added to PRMS to account for discharge to tile-drains under saturated soil conditions. Thresholds and other irrigation model parameters are dynamically applied to match water takings for each crop type and subwatershed under average and dry year conditions. While the primary benefit is a fully integrated approach, components of the Farm Package are also improved through integration with the more complete PRMS soil zone hydrology code. This extended version of GSFLOW has been tested in the Whitemans Creek watershed in southwestern Ontario, Canada. Whitemans Creek drains an area of approximately 400 km² (150 square miles), with actively-cultivated agricultural fields comprising 60% of the watershed. The upper portion of the watershed is extensively tile drained, while the central and lower portions of the watershed contain significant wetlands and groundwater supported fish habitat. Total permitted takings, primarily from drought sensitive shallow sand aquifers in the lower watershed, exceed 80,000 m3/d. The model simulates groundwater heads, streamflow, and wetland stage on a daily basis under current conditions, along with future climate, drought and development scenarios. The effect of agricultural demand-driven pumping on specific sensitive stream reaches is fully represented. Detailed breakdowns of water budget components, aggregated on a monthly and annual basis, provide insight into future development scenarios. Results of the model will be utilized in the future to better manage water allocation under drought conditions.