Sensitivity of agricultural runoff loads to rising levels of CO2 and climate change in the San Joaquin Valley watershed of California
Introduction
The general consensus of atmospheric scientists is that the earth's temperature is increasing (IPCC, 2007), and as global temperatures increase the hydrologic cycle is becoming more dynamic. Predicted global mean temperature in 2100 will be between 1.1 and 6.4 °C higher than in 1990 with additional changes in rainfall intensity and quantity (IPCC, 2007). Analyses made by leading climate research centers indicate that the global mean surface temperature in 2006 was 0.42–0.54 °C above the 1961–1990 annual average (WMO, 2006). For the next two decades, the Intergovernmental Panel on Climate Change (IPCC, 2007) states that a warming of about 0.2 °C per decade is projected for a range of IPCC emission scenarios. Even if the concentrations of all greenhouse gases and aerosols were to be kept constant at year 2000 levels, a further warming of about 0.1 °C per decade would be expected. Global Climate Models (GCMs) indicate that it is very likely (greater than 90% probability) that heat extremes, heat waves, and heavy precipitation events will become more frequent (IPCC, 2007), and an overall increase in global precipitation will occur. For the state of California, GCM predictions of precipitation vary widely, with both increases and decreases being projected (e.g., Smith and Mendelsohn, 2007). This leads to a lack of confidence in the stability of regional and seasonal patterns of precipitation, implying the possibility of changes to the hydrologic cycle. Even slight changes in precipitation and hydrological conditions can potentially affect crop production and agricultural runoff in highly agricultural watersheds.
Increasing agricultural contamination of surface waters has generated substantial concern since the 1940s (Larson et al., 1995). This concern is especially pertinent in the highly agricultural San Joaquin River watershed in California. This watershed, along with the Sacramento River Watershed, drains into the Sacramento–San Joaquin Delta (Delta), which in recent years has seen an appreciable decline in aquatic species, attributed in part to an increase in water toxicity levels (Werner et al., 1999). Principal contaminant sources to the Delta include agricultural and urban runoff, discharges from abandoned mines, and point source discharges. Detections of agricultural runoff have been reported in the Delta and upstream source waters (e.g., Dileanis et al., 2002, Guo et al., 2004, Weston et al., 2004, Amweg et al., 2006; CVRWQCB, 2006). Pesticide detection frequency in surface waters has become a major concern, as California contains approximately 2–3% of the nation's agricultural land, yet accounts for 25% of the nation's pesticide use (Kegley et al., 2000). In agricultural regions such as the San Joaquin River watershed the primary mode of agricultural non-point source pollution transport is sediment and water runoff (Leonard, 1990). Changes in climate, which impact agricultural pollutant transport through water and sediment runoff, will thus directly affect future levels of water quality in the Sacramento–San Joaquin watersheds and the Delta.
Much research has been done on agricultural runoff (e.g., Griffin and Bromley, 1982). However, insufficient work has been done to examine the effects of imminent climatic changes on agricultural runoff (e.g., Panagoulia, 1991, Arnell, 1992, Murdoch et al., 2000). Mander et al. (2000) showed that the contaminant concentrations in agricultural runoff (total-N, total-P, SO4, and organic material) have decreased in recent years (1987–1997). Chaplot (2007) examined the effects of increasing CO2 concentrations, rainfall intensity, and surface air temperature on nitrate runoff, finding that atmospheric CO2 concentration was the main controlling factor for nitrate yield. Studying the impacts of climate change in the southeastern United States, Cruise and LimayeNassim Al-Abed (1999) showed that several watersheds exhibited high nitrogen levels in runoff. Tong et al. (2007) discovered that the probability of eutrophication is likely to increase in future climatic conditions. Hanratty and Stefan (1998) examined the effect of climate change on quality and quantity of runoff from a Minnesota agricultural watershed and found a decrease in mean annual streamflow, nutrient, and sediment yield. To date, there has only been one qualitative study on the impacts of climate change on pesticide fate and transport in the context of environmental protection (Bloomfield et al., 2006). No quantitative estimates of this effect are currently available.
The Soil and Water Assessment Tool (SWAT) (Arnold et al., 1998) watershed model was chosen for this study. SWAT includes algorithms for predicting how CO2 concentration, precipitation, temperature, and humidity affect plant growth, evapotranspiration (ET), snow, and runoff generation. SWAT, therefore, is an effective tool for investigating climate change effects. Several case studies of climate change impacts on water resources have applied SWAT (e.g., Hanratty and Stefan, 1998, Rosenberg et al., 1999, Cruise and LimayeNassim Al-Abed, 1999, Stonefelt et al., 2000, Fontaine et al., 2001, Eckhardt and Ulbrich, 2003, Chaplot, 2007, Schuol et al., 2008). SWAT has also been used to model portions of the San Joaquin watershed (Flay and Narasimhan, 2000, Luo et al., 2008). This study, however, marks the first time SWAT has been used to model agricultural runoff in the San Joaquin watershed under a changing climate.
Despite many climate change studies, up-to-date quantitative information on the effects of the changes of precipitation and temperature on soil and water resources is still scarce. The objective of this study is to quantify the effects that climate change will have on the fate of agricultural pollutants and transport of such substances within a highly agricultural region in California's Central Valley. For the study, a SWAT model of the San Joaquin watershed in California (Luo et al., 2008) was used to assess the impacts of climate change on the fate and transport of agricultural pollutants. Different scenarios of precipitation (0%, 10%, and 20% increase or decrease in precipitation amount and average daily rainfall intensity), surface air temperature (a 1.1 °C or 6.4 °C increase from current climate), and an increase of CO2 concentration from the present-day concentration of 330 ppm to an extreme IPCC prediction of 970 ppm were tested using SWAT. Long-term estimates of sediment, fertilizer (nitrate and total phosphorus), and pesticide (diazinon and chlorpyrifos) yields were compared to a benchmark scenario with a CO2 concentration of 330 ppm and a present-day reference climate.
Section snippets
Site description
The San Joaquin River watershed was selected for this study (Fig. 1). The watershed area is 14,976 km2 and includes the counties of San Joaquin, Calaveras, Stanislaus, Tuolumne, Merced, Mariposa, Madera, and Fresno. Latitude and longitude range from 36°30′N to 38°50′N and from 119°45′W to 121°30′W, respectively. The United States Geological Survey (USGS) river monitoring site at Vernalis (USGS #11303500) was chosen as the outlet for the entire watershed. The discharge inlets of the upper San
Climate characteristics of the reference scenario
The average simulated yearly rainfall for all climate stations during the simulated 100-year reference period was 295.4 mm, approximately 17 mm greater than the observed regional CIMIS average. The 100-year minimum and maximum simulated yearly precipitation amounts were 127.5 and 541.8 mm, respectively. The largest amount of precipitation simulated for a single day was 100.2 mm. The average minimum and maximum daily temperature was 7.78 and 23.8 °C, respectively.
Impact of climate and atmospheric CO2 concentration changes on agricultural pollutant yields
The climate change sensitivity
Simulation results
All agricultural runoff components were significantly affected by precipitation changes and to a lesser degree by changes in temperature and CO2 concentration. The relationship between precipitation changes and increased agricultural pollutant runoff is easily understood, as rainfall impacts and runoff are the driving mechanisms for pollutant transport within watersheds.
Nitrate and total phosphorus losses depend on the hydrologic balance, the quantities present in the soil either from natural
Conclusions
This study illustrates changes in agricultural runoff related to potential climate change based on SWAT model simulations in an agriculturally dominated area of the San Joaquin River watershed. The results indicate that the hydrological system in the study area is very sensitive to climatic variations on an annual basis and/or over a long time period. As expected, precipitation had a greater impact on agricultural runoff compared to changes in either CO2 or temperature. Changes in precipitation
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