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Using Deficit Irrigation to Reduce Vulnerability in Food-Energy-Water Nexus

A new publication by 2020-21 Global Change Fellow Hemant Kumar and SE CASC Faculty Affiliate Sankar Arumugam was published June 12, 2023 in the journal, Water Resources Research. The article, Understanding the Food-Energy-Water Nexus in Mixed Irrigation Regimes using a Regional Hydroeconomic Optimization Modeling Framework, summarizes research by the Climate, Hydrology and Water Resources Modeling and Synthesis Group at NCSU. The following summary was written by Hemant Kumar.

The food-energy-water (FEW) nexus refers to the interconnectedness of food, energy, and water systems. For instance, 85% of global water withdrawals are used for food production and 16% of United States’ national energy budget goes towards food production and processing. The supplies of these interconnected resources are becoming less secure in the changing climate especially since global demand continues to increase. It is important to analyze the water ↔ food and energy ↔ food linkages in agriculture-oriented economies as is the case in the southern Flint River Basin in Georgia in southeastern US where agriculture contributes about $6 billion in direct and indirect benefits annually.   

Agriculture in the southeastern US has historically been rainfed (farming that relies on rainfall or water) but has been experiencing a steady increase in irrigated acreage in recent decades. Agricultural irrigation comprises around 90% of the total water needs of the region during the April-September growing season. More than 70% of the irrigation (403,000 acres) relies on groundwater from the Floridian aquifer. This changing mixed irrigation pattern combined with the increasing urbanization and population has intensified the competition and conflict for water both within the basin as well across the basin. The Georgia Environmental Protection Division has had to suspend issuing new permits for groundwater pumps and pay farmers not to irrigate during long droughts (most recently in 2013). The Division had taken more than 33,000 acres out of irrigation through partly successful bidding for a total cost of approximately $4.5 million in 2001.

We utilized hydroeconomic models to analyze the FEW nexus. Hydroeconomic models link agricultural inputs (rainfall, labor, seeds, machinery, etc.) to agricultural outputs (crop yield). Estimating the crop yield (bushels/acre) is an essential but difficult task of developing a model. Existing approaches use different approaches such as statistical regression and look up data tables. However, such approaches are limited in their capability to model the effect of intra-seasonal climatic conditions on crop yield and to easily extend the model to other regions. The novelty of our work lies in using a biophysical crop growth model to estimate the season’s crop yield based on the daily climatic conditions (precipitation, temperature, evapotranspiration) throughout the season. The integration of biophysical crop models and profit optimization modules so far has been limited by the following: frequent calls between optimization and crop simulation module slows the optimization as the crop simulation module operates at daily scale and thus runs slower compared to optimization module. We have solved this integration problem by developing a novel Bayesian Hierarchical Model which drastically reduces the optimization time.

Our study shows that rainfed and deficit irrigation are preferred over full irrigation in the mixed irrigation regime of the region as the gains in crop yield with additional supplied water diminish while the pumping cost increases due to deeper groundwater table in drier years. For example, the inclusion of deficit irrigation while maintaining crop diversity increased the revenue by 18% and profit by 38% compared to the existing scenario while simultaneously reducing the amount of total annual water pumped by 14% (245,000 acre-feet annually). The proposed approach uses nationally available datasets from USDA MASS and USGS and open-source AquaCropOS model and thus can be extended to other regions. We expect the regional water managers would utilize the proposed RHEO framework in reducing the amount and cost of groundwater withdrawals by considering deficit irrigation strategies during drought periods and thereby reducing the FEW vulnerability of the region.

Figure 3. The conceptual model of deficit irrigation: crop yield under deficit irrigation (yellow lines) varies between yield under rainfed (blue line) and full irrigation (orange line) for different values of deficit irrigation parameters (α and I).

Kumar, H., Zhu, T., & Sankarasubramanian, A. (2023). Understanding the food-energy-water nexus in mixed irrigation regimes using a regional hydroeconomic optimization modeling framework. Water Resources Research, 59, e2022WR033691.