Applying system dynamics modeling for assessing environmental-economic sustainability of regional agriculture: An application for the Free State, South Africa
Authors: Bogra Shelly 
Published: August 2023

Abstract
Agriculture is the predominant user of both, freshwater and land. With rising population leading to increasing demand for food, the impacts of water-intensive food production in view of constrained soil fertility, limited supply of land along with increasing energy costs of irrigation and labor, may make regional cropping patterns unsustainable. Additionally, water-scarce regions may face additional challenges to meet the requirements of local agriculture. To assess the sustainability of regional agriculture, a generic model is developed using Vensim PLE software for system dynamics modeling (SDM). Considering agriculture as a complex system, the developed model captures the causality between various components of an agricultural system in the form of a causal loop diagram (CLD) and, further simulates environmental-economic costs of rain-fed, irrigated, and rain-water harvested agricultural-systems, through quantitative, stock and flow (SFD) model. Using crop-specific data of quantity produced, area harvested, and crop-water-requirements (CWR), the SFD model is capable of simulating water demand, energy requirement, and monetary return (MR), on the inputs used. The developed SFD model is further extended to simulate the potential impacts of infield-rain-water-harvesting structures (IRWHS). To support its applicability for regional assessments, it is calibrated and validated for a water-stressed region, namely, the Free State, South Africa. For the Free State, the model is simulated till 2035, with and without IRWHS, for four crops (namely, potato, wheat, maize-white, and maize-yellow). Further, crop-specific data required for the simulation (production, area, yield, etc.) is obtained for the period 1990-2010 and, CWR data is obtained from the Water Footprint Network (WFN)'s database. IRWHS implementation is simulated starting in 2011, with the assumption that IRWHS leads to a 50% increase in the productivity of considered crops. The model was validated for both, behavior and structure. Additionally, to assess the robustness of simulated results against the available data, the simulated results were statistically analyzed using regression techniques. For the Free State, the results indicate that due to the least area and CWR, potato offers the highest MR on irrigation-energy and water-used, with or without IRWHS in both, irrigated and rain-fed systems. Further, MR on water used in irrigated areas is higher than in the rain-fed-areas. Additionally, the greatest beneficiary is expected to be maize-yellow with IRWHS, followed by maize-white, wheat, and potato. Furthermore, effective MR is expected to be lower for green water (rainwater available as soil moisture), except for wheat. The biggest revelation from the simulated results is that various crops using different amounts of water and land offer unequal MR. That is, simulated results indicate variability in MR, for both, water and electricity, used for different crops. Thus, though other studies exploring rainwater-harvesting systems indicate that water-harvesting has both, monetary and productivity, benefits; this simulation clearly shows that crop-wise-benefits are not equal. These results indicate that the crop which has the least water requirement (i.e. potato) may offer the highest MR on electricity and blue water (water from surface and ground water-bodies), whereas, the highest MR on water used under harvesting structure is offered by crops that have higher crop-water-requirements (wheat and maize, in the present study). Since generic, this model can assess regional-specific environmental-economic costs of different agricultural-systems, and therefore, can assist in the selection of sustainable cropping patterns.