Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2009
Publication Date: 9/21/2009
Citation: Tomer, M.D., Schilling, K.E. 2009. A Simple Approach to Distinguish Land-use and Climate-change Effects on Watershed Hydrology. Journal of Hydrology. 376(1-2):24-33. Interpretive Summary: Impacts of climate change on watershed hydrology are subtle compared to cycles of drought and surplus precipitation (PPT), and difficult to separate from effects of land-use change. In the US Midwest, observed hydrologic trends have been more attributed to changing agricultural land use than to climate change. These agricultural changes have also led to increased fertilizer use and nutrient losses, and contributed to Gulf of Mexico hypoxia. In a 25-year, small-watershed experiment in Iowa, an ecohydrologic analysis showed effects of agricultural practices and climate change on hydrology could be separated. Both effects resulted in increased stream discharge as a fraction of precipitation. However, the proportion of evaporative demand that was unmet increased under conservation practices, but decreased with time as the ratio of water supply: water demand (that is, precipitation:evaporative demand) increased. Longer-duration records from four large Midwest watersheds showed at least three of four watersheds had significant decreases in evaporative demand, and increases in precipitation, discharge, baseflow and ratio of precipitation: evaporative demand (p<0.10). The fraction of precipitation discharged (unused) increased and unsatisfied evaporative demand generally decreased in all four watersheds, indicating the effect of climate change on hydrology was greater than that of land use, particularly since the 1970s. Agricultural changes were associated with ecohydrologic shifts that affected the timing and significance, but not direction, of these trends. Results suggest climate change has increased discharge from Midwest watersheds, thus exacerbated Gulf of Mexico hypoxia by making agricultural nutrients more susceptible to loss. This information is useful to agricultural policy makers, scientists, service agencies, and producers, who need to recognize that recent climate trends in the Midwest pose increasing challenges for conservation of nutrients in agricultural production systems.
Technical Abstract: Impacts of climate change on watershed hydrology are subtle compared to cycles of drought and surplus precipitation (PPT), and difficult to separate from effects of land-use change. In the U.S. Midwest, increasing baseflow has been more attributed to increased annual cropping than climate change. The agricultural changes have led to increased fertilizer use and nutrient losses, contributing to Gulf of Mexico hypoxia. In a 25-year, small-watershed experiment in Iowa, when annual hydrologic budgets were accrued between droughts, a coupled water-energy budget (ecohydrologic) analysis showed effects of tillage and climate on hydrology could be distinguished. The fraction of PPT discharged increased with conservation tillage and time. However, unsatisfied evaporative demand (PET – Hargreaves method) increased under conservation tillage, but decreased with time. A conceptual model was developed and a similar analysis conducted on long-term (>1920s) records from four large, agricultural Midwest watersheds underlain by fine-grained tills. At least three of four watersheds showed decreases in PET, and increases in PPT, discharge, baseflow and PPT:PET ratios (p<0.10). An analysis of covariance showed the fraction of precipitation discharged increased, while unsatisfied evaporative demand decreased with time among the four watersheds (p<0.001). Within watersheds, agricultural changes were associated with ecohydrologic shifts that affected timing and significance, but not direction, of these trends. Thus, an ecohydrologic concept derived from small-watershed research, when regionally applied, suggests climate change has increased discharge from Midwest watersheds, especially since the 1970s. By inference, climate change has increased susceptibility of nutrients to water transport, exacerbating Gulf of Mexico hypoxia.