Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 9/28/2009
Publication Date: 12/11/2009
Citation: Tomer, M.D., Schilling, K. 2009. Discerning Climate and Land-use Change Impacts on Watershed Hydrology: Implications for Gulf Hypoxia [abstract]. Soil & Water Conservation Society (SWCS) Science 2 Solutions Conference. p. 51. Interpretive Summary:
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 (eco-hydrologic) analysis showed effects of tillage and climate on hydrology could be distinguished. The fraction of PPT discharged increased with conservation tillage and time. However, the fraction of evaporative demand (PET – Hargreaves) that was unsatisfied increased under conservation tillage, but decreased with time. A conceptual model was developed, illustrating that changes in ET associated with land-use change create monotonic shifts in the proportions of available water and energy that are unsatisfied, while climate change that shifts PPT:PET ratios causes contrasting shifts in those proportions. To extend this concept beyond experimental watersheds, a similar analysis conducted on long-term (>1920s) records from four large, agricultural Midwest watersheds underlain by fine-grained tills. At least three of these four watersheds showed decreases in PET, and increases in PPT, discharge, baseflow (p<0.10), and all four showed increases in PPT:PET ratios (p<0.05). 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); the conceptual model suggests such eco-hydrologic shifts are climate driven. Within watersheds, agricultural changes were associated with eco-hydrologic shifts expected with decreasing ET; but these only affected timing and significance, but not direction, of the dominant climate-driven 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. This conclusion expands on evidence from other recent studies, which have suggested climate trends in the Midwest will require that conservation systems in this region be improved and better promoted.