Quantifying future water resources availability and agricultural productivity in agro-urban river basins
Date
2022
Authors
Aliyari, Fatemeh, author
Bailey, Ryan T., advisor
Arabi, Mazdak, committee member
Bhaskar, Aditi, committee member
Sanford, William E., committee member
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Abstract
Climate change can have an adverse effect on agricultural productivity and water availability in semi-arid regions, as decreases in surface water availability can lead to groundwater depletion and resultant losses in crop yield due to reduced water for irrigation. Competition between urban and agricultural areas intensifies groundwater exploitation as surface water rights are sold to growing municipalities. These inter-relationships necessitate an integrated management approach for surface water, groundwater, and crop yield as a holistic system. This dissertation provides a novel integrated hydrologic modeling approach to quantify future water resources and agricultural productivity in agro-urban river basins, particularly in arid and semi-arid regions where surface water and groundwater are managed conjunctively to sustain urban areas and food production capacity. This is accomplished by i) developing an integrated hydrologic modeling code that accounts for groundwater and surface water processes and exchanges in large regional-scale managed river basins, and demonstrating its use and performance in the economically diverse South Platte River Basin (SPRB), a 72,000 km2 river basin located primarily in the state of Colorado, USA; ii) using the model to understand possible future impacts imposed by climate variation on water resources (surface water and groundwater) and agricultural productivity; and iii) quantifying the combination impacts of agriculture-to-urban water trading and climate change on groundwater resources within the basin. This dissertation presents an updated version of SWAT-MODFLOW that allows application to large agro-urban river basins in semi-arid regions. SWAT provides land surface hydrologic and crop yield modeling, whereas MODFLOW provides subsurface hydrologic modeling. Specific code changes include linkage between MODFLOW pumping cells and SWAT HRUs for groundwater irrigation and joint groundwater and surface water irrigation routines. This conjunctive use, basin-scale long-term water resources, and crop yield modeling tool can be used to assess future water and agricultural management for large river basins across the world. The updated modeling code is applied to the South Platte River Basin, with model results tested against streamflow, groundwater head, and crop yield throughout the basin. To assess the climate change impacts on water resources and agricultural productivity, the coupled SWAT-MODFLOW modeling code is forced with five different CMIP5 climate models downscaled by Multivariate Adaptive Constructed Analogs (MACA), each for two climate scenarios, RCP4.5, and RCP8.5, for 1980-2100. In all climate models and emission scenarios, an increase of 3 to 5 °C in annual average temperature is projected by the end of the 21st century, whereas variation in projected precipitation depends on topography and distance from the mountains. Based on the results of this study, the worst-case climate model in the basin is IPSL-CM5A-MR-8.5. Under this climate scenario, for a 1 °C increase in temperature and the 1.3% reduction in annual precipitation, the basin will experience an 8.5% decrease in stream discharge, 2-5% decline in groundwater storage, and 11% reduction in crop yield. In recent decades, there has been a growing realization that developing additional water supplies to address new demands is not feasible. Instead, managing existing water supplies through reallocations is necessary to tackle water scarcity and climate change. However, third-party effects associated to water transfers has limited the growing water market. This study also quantifies the combination impacts of agriculture-to-urban water trading (widely known as 'buy and dry') and climate change on groundwater availability in semi-arid river basins through the end of 21st century, as groundwater pumping increases to satisfy irrigation water lost to the urban sector. For this analysis, we use the hydrological modeling tool SWAT-MODFLOW, forced by projected water trading amounts and two downscaled GCM climate models, each for two emission scenarios, RCP4.5 and 8.5. According to the results of this study, agriculture-to-urban water trading imposes an additional basin-wide 2% reduction in groundwater storage, as compared to changes due to climate. However, groundwater storage changes for local subbasins can be up to 8% and 10% through the mid-century and end of the century, respectively.
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Subject
hydrological modeling
surface water - groundwater interaction
water trading
large scale watershed
climate change
SWAT-MODFLOW