Effects of Climate Change on Irrigation Decisions and Low Flow Frequency for a Typical Agricultural River Basin of the Midwestern US

The Midwest is the largest agricultural area of the United States. Historically, the climate, characterized by moderate temperatures and ample rainfall, has been suitable for un-irrigated agriculture. However, the specter of climate change has created concerns about the future of Midwestern agriculture, regional fresh water resources and the relationship between the two. Implications of climate change for the Midwest are revealed at the river basin scale in a recent study on the Mackinaw River Basin, a typical agricultural Midwestern watershed of central Illinois, through modeling exercises. Generally in this study a future climate with more frequent droughts is envisioned based on the outcome of one of the major General Circulation Models (GCMs), the Canadian Climate Centre model. Climate change in and of itself will affect the vulnerability of regional fresh water resources, by altering the low flow frequencies of stream at reference gauging stations. Moreover, the threats of droughts may motivate farmers to introduce irrigation in this traditionally rain-fed area to maintain high and stable yields. Such irrigations, if any, could exacerbate the effects of the changes in climatic factors. This study shows that the changes in climatic factors of temperature and precipitation will reduce the agricultural productivity, trigger irrigation and increase the low flow frequencies at reference gauging stations. However, the adverse effects of changes in temperature and precipitation may well be counteracted by the effects of elevated CO2 concentration in atmosphere. Thus, the opposing effects of climate change could very well leave agriculture in central Illinois more or less unchanged. Introduction Over the past few years the reality of climate change came to be well-established by theory and observation. There exists much scientific evidence indicating that our planet is undergoing general warming accompanied by shifts in other climatic factors (IPCC, 2001). Copyright ASCE 2004 World Water Congress 2004 2 The Midwest is the largest agricultural area of the United States. Historically, the climate there has been suitable for un-irrigated agriculture, with moderate temperatures and ample rainfall. The region also possesses abundant fresh water resources consisting of rivers and lakes, which host diverse aquatic ecosystems. However, the specter of climate change has created concerns about its effects—both positive and negative—on Midwestern agricultural economy, regional water resources and the relationship between the two. This paper presents our findings of a recent study of climate change impacts on the Mackinaw River Basin, a typical agricultural Midwestern watershed of central Illinois, through modeling exercises. Generally in this study we envision a future climate with more frequent droughts on the basis of the predictions of one of the major General Circulation Models (GCMs), the Canadian Climate Centre model. Such climate change in and of itself will influence the agricultural productivity and hydrological conditions of the study watershed not only directly but also by altering the current practices in agriculture. The threats of droughts in the climate pattern we envision may motivate farmers to introduce irrigation in this traditionally rain-fed area to maintain high and stable yields. Such irrigation, if any, could exacerbate the effects of the changes in climatic factors. In this study, we use SWAT (Soil and Water Assessment Tool) to investigate these potential consequences of climate changes on the Mackinaw River Basin. SWAT is a physically-based watershed scale model developed for the simulation of watershed hydrology and the evaluation of long-run effects of land management practices (Arnold et al., 1998). The model has the capability to simulate water movement, vegetation growth and a range of operations, including irrigation, occurring in a typical agricultural watershed. The simulation within SWAT is driven by meteorological inputs, such as temperature, precipitation and CO2 concentration in atmosphere. By modifying these input data, various climate patterns can be represented. The effects of climate change on fresh water resources is characterized by the low flow frequencies of the stream at reference gauging stations. The low flow standards, here, are assumed to be the historical 7-day 10-year low flow (7Q10). An economic analysis is included in our assessment framework. The economic analysis helps determine the economic viability of irrigation decisions under various climatic conditions and reveals the interactions between the agricultural economy and regional water resources under climate change. Following the introduction, the methods we use in the modeling exercises are described and the results and conclusions are presented. Copyright ASCE 2004 World Water Congress 2004 3 Methods The study watershed of the Mackinaw River Basin (USGS Cataloging unit: 07130004) is located in central Illinois. The river basin drains 3000 km and more than 95% of the area is for agricultural land use. The SWAT modeling exercise starts with watershed delineation. SWAT2000, the version of SWAT we use here, has been incorporated into the BASINS (Better Assessment Science Integrating Point and Nonpoint Sources) version 3.0 software, which provides GIS-based watershed delineation tools that allows the delineation process to be completed automatically with the desired sensitivity. The data required for the delineation are included with the BASINS software package. In our modeling exercises, the entire Mackinaw River Basin is portioned into 19 subbasins (Figure 1). Two U.S. Geological Survey (USGS) gauges, one near Congerville (USGS site number: 05567500) and the other near Green Valley(USGS site number: 5568000) are chosen as references to calculate the frequencies of low flow events. The 7Q10 at the two gauges are 0.033 m/s and 0.714m/s respectively(Singh et al., 1988).