Cache Forecast Scenario Groundwater-Flow Models

The Mississippi Alluvial Plain (MAP) is one of the most important agricultural regions in the United States and underlies about 32,000 square miles of Missouri, Kentucky, Tennessee, Mississippi, Louisiana, and Arkansas. The MAP region supports a multibillion-dollar agricultural industry. The MAP is part of the Mississippi Embayment with several water-bearing units that make up the Mississippi Embayment Regional Aquifer System (MERAS). These water bearing units include the Mississippi River Valley Alluvial aquifer, Claiborne aquifers, and Wilcox aquifers. In northeastern Arkansas, the Cache area has been designated as a critical groundwater area (CGWA) because of decades of groundwater declines that resulted from past and current water use. The objective of the report associated with this data release is to document and describe the construction, simulation, and results of scenario forecasts run for the Cache scenario model(s) with a focus on results within the designated Cache CGWA. The calibrated Cache model from Traylor and others (2024) was used as the base model for each forecast scenario model. Scenarios for the Cache model included seven climate scenarios, five global groundwater pumping reduction scenarios, and four groundwater pumping reduction scenarios by crop and location. Each scenario was outlined by the researchers and managers at U.S. Department of Agriculture (USDA) Agricultural Research Service, Natural Resource Conservation Service, Bayou Meto Water Management District, and White River Irrigation District to target and simulate the expected reductions in groundwater pumping that could result from the adoption of various on-farm conservation practices. The scenarios modeled do not simulate the conservation practices, but only the expected reductions in groundwater pumping from the implementation of the conservation practices. The climate scenarios were chosen from an ensemble of 32 Coupled Model Intercomparison Project Phase 5 (CMIP5) Localized Constructed Analogs downscaled Global Climate Models (GCMs) for the Representative Concentration Pathway 8.5 (RCP8.5; van Vuuren, 2011). A MERAS extent Soil Water Balance (SWB) model simulated recharge and irrigation water use using the 32 CMIP5 downscaled RCP8.5 GCM precipitation and temperature data as direct inputs (Nielson and Westenbroek, 2023; Villers and Ladd, 2023). Those seven GCMs were ccsm4_rcp85, cesm1_bgc_rcp85, cesm1_cam5_rcp85, cmcc_cm_rcp85, cmcc_cms_rcp85, cnrm_cm5_rcp85, mpi_esm_lr_rcp85. The management scenarios included a suite of round 1 and round 2 simulations where round 1 scenarios were developed and run before the round 2 scenarios. Round 1 management scenarios included a global reduction in agricultural groundwater pumping (all crop types: aquaculture, corn, cotton, other crops, rice, and soybean) across the entire Cache and Grand Prairie model domains by 10, 20, 30, 40, and 50 percent, respectively. For the Cache model, round 2 consisted of eight management scenarios that explored the impacts of reductions in groundwater pumping tied to (a) irrigation savings assumed for each of the specific crops grown in 2020, (b) irrigation savings applied inside and outside of the CGWAs, and (c) on-farm irrigation reservoirs evaluated at two reservoir construction rates. For Round 2 scenarios 1-8, specific crop types, harvest areas, and locations were based on the 2020 USDA NASS cropland data layer (USDA NASS, 2020).

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