UNDERSTANDING THE WEATHER- AND SOIL-RELATED VARIABILITY IN AGRICULTURAL WATER FOOTPRINTS: CASE STUDY OF MAIZE AND SOYBEANS GROWN IN THE ST. JOSEPH WATERSHED

Abstract

Introduced in 2002, the water footprint is a valuable tool for understanding the consumption of freshwater resources. The traditional approach to quantifying the water footprint is to sum the green, blue, and gray water footprint components. The green water footprint is the volume of water that comes from precipitation, is stored in the soil, and used by vegetation. The blue water footprint is the volume of surface or ground water that is withdrawn and applied to cultivated lands via irrigation. These components are based on the evapotranspiration of green and blue water resources, respectively. The gray water footprint is the volume of water that is needed to dilute the resultant pollutants to ensure compliance with water quality standards in natural water bodies. Though applicable to many products and at a range of scales, the water footprint methodology is most commonly applied to agricultural products. This is because of the large volumes of water that products from this industry require. For example, in the U.S., maize and soybeans require 190 x 109 and 120 x 109 m3 in total water per year. Due to the large demands of water, applying the water footprint to an area with intensive agriculture is beneficial to effective resource management, even in areas with abundant water resources such as the Great Lakes Basin, which contains 21% of the world’s freshwater resources. Thus, the St. Joseph watershed, which is located in this area, will be investigated for how maize and soybean produced affect the freshwater resources. Because of the lack of insight gained from the traditional water footprint regarding the effects that water depletion has on local landscapes, a new approach is proposed, called the relative water footprint. Using this approach, it is shown that relative to a natural landscape of woody vegetation and deciduous trees, the agricultural water demands of the St. Joseph watershed are greatly reduced, at least in terms of the green water footprint. However, the blue and gray water footprints for maize and soybeans may still be significant and highly variable, with blue water footprints dependent on rainfall patterns, soil types, and irrigation scheduling, and gray water footprints dependent on fertilizer application rates.