Water and soil resources response to rising levels of atmospheric CO2 concentration and to changes in precipitation and air temperature

The quantification of the diverse responses of soils and terrestrial fresh water to elevated levels of greenhouse gases and to climate change is crucial for the proper management of natural ecosystems in the future. Despite previous experiments and simulations, there is still a need for the evaluation of the impact of these global changes at the watershed level. The main objective of this paper is to examine the effects of increasing CO2 concentrations and rainfall changes associated with changes in average daily rainfall intensity, and surface air temperature on loads of water, NO3-N and sediments from watersheds exhibiting different environmental conditions. The interactively coupled (CO2-climate-landuse) Soil and Water Assessment Tool (SWAT) was used to predict the effect of variations in precipitation and rainfall intensity (a 10%, 20% and 40% increase or decrease) or surface air temperature (an increase of 0.5 and 3.5 degrees C in the mean winter temperature) associated with an increase Of CO2 concentration from 330 to 950 ppm in two agricultural watersheds in Iowa and Texas. Over a 100-year simulated period: (1) precipitation changes primarily affected flow and sediment discharges white temperature and changes in atmospheric CO2 concentration had a smaller effect; (2) CO2 concentration was the main controlling factor of NO3-N loads; and (3) global changes in the humid watershed had a greater effect on the water and soil resources than in the semi-arid. In particular, increasing precipitations in the humid watershed significantly increased the median and interquartile of outputs of all the variables under consideration. In contrast, changes in surface air temperature had only a slight impact on loads. However higher temperatures in the more humid site tended to reduce the highest loads. These results demonstrate that changes in CO2 concentration and climate, particularly an increase in precipitation, had significant effects on the soil and fresh water resources. The magnitude of these effects differed according to the watershed's characteristics. Finally, the general applicability of these results and the limits of the approach are discussed