Comparative study of climate and human impacts on seasonal baseflow in urban and agricultural watersheds

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 37,, doi: /2009gl041879, 2010 Comparative study of climate and human impacts on seasonal baseflow in urban and agricultural watersheds Dingbao Wang 1 and Ximing Cai 1 Received 23 November 2009; revised 15 February 2010; accepted 18 February 2010; published 19 March [1] This study explores the long term trends of low flow magnitude and the slopes and shapes of the recession curves during winter and summer seasons under climatic and human factors. Four watersheds in the American Midwest are selected for the analysis, including two urban watersheds (Salt Creek and Des Plaines) and two agricultural watersheds (Embarras and Kankakee). The results show that the long term baseflow recession slope trends in all the watersheds are primarily induced by human interferences. In the urban watersheds, the recession slopes decrease over time in both winter and summer due to effluent discharges. In the Kankakee watershed with irrigation, the recession slopes decrease in winter but increase in summer, and the opposite winter and summer trends are caused by the seasonal water use regime of irrigated agriculture. In the Embarras watershed with rainfed agriculture, the recession slopes decrease over time in winter but display no change in summer. Sources of water withdrawal (groundwater versus surface water) also have different impacts on the recession process. This long term analysis of recession rates, in conjunction with the changes in low flow magnitude, offers valuable insight on human interferences to hydrologic processes. Beyond the specific case studies, this paper documents how a scientific approach based on existing streamflow observation can be applied to improving our understanding of the impact of human and climatic influences on baseflow and low flow processes. Citation: Wang, D., and X. Cai (2010), Comparative study of climate and human impacts on seasonal baseflow in urban and agricultural watersheds, Geophys. Res. Lett., 37,, doi: /2009gl Introduction [2] The impacts of climate change and human interferences, such as land use and land cover change, on streamflow have been studied in many watersheds around the world. The hydrologic trends resulting from these impacts are usually analyzed using simulation models and historical discharge. Many of these studies, motivated by the importance of low flows for both human and ecological water demands [e.g., Schilling et al., 2008], have explored the impact of climate change and human activities on baseflow. For example, St. Jacques and Sauchyn [2009] observed that the thawing of permafrost resulting from climate warming in the Northwest Territories augmented groundwater discharges into streams. Zhang and Schilling [2006] found that 1 Department of Civil and Environmental Engineering, University of Illinois at Urbana Champaign, Urbana, Illinois, USA. Copyright 2010 by the American Geophysical Union /10/2009GL the conversion of perennial vegetation to seasonal row crops in the Mississippi River Basin reduced annual evapotranspiration (ET) and increased groundwater recharge. These studies focused on the magnitude of discharge. Most recently, Wang and Cai [2009] incorporated human activities into the base flow recession analysis and applied this analysis to an urban watershed (Salt Creek) in the winter season (when the natural ET can be ignored), to estimate groundwater pumping and return flow. [3] This study explores the long term trends of low flow magnitudes, slopes and shapes of the baseflow recession curves during winter and summer seasons under climatic and human influences. Four watersheds in the American Midwest are selected for the analysis, including two urban watersheds (Salt Creek and Des Plaines) and two agricultural watersheds (Kankakee and Embarras). It is demonstrated that such analysis provides additional insights into the causes (climate versus human) of the baseflow alterations underlying different land use conditions (urban versus agricultural). Furthermore this study diagnoses the causes of the observed recession slope trends by extending the method presented by Wang and Cai [2009], i.e., including surface water withdrawal and water diversion explicitly into the analysis and applying to summer seasons and agricultural watersheds. The overall purpose of this paper is to detect the trends of baseflow recession slopes in urban and agricultural watersheds and diagnose the climatic and human factors driving the trends. Although this paper only presents some minor extensions to the method introduced by Wang and Cai [2009], these extensions applied to several urban and agricultural watersheds enable us to sort out some complex interactions among land use, water use and return flows, municipal and agricultural practices, climate, and baseflow responses. 2. Methodology [4] Due to the variability of evaporation losses and recharge rates, individual recession segments may have different slopes [Posavec et al., 2006]. Master recession curve (MRC) is usually used to compile individual recessions. In this study, a Visual Basic program is used to construct the MRC [Posavec et al., 2006]. To study the variability in a MRC, the recession slope (K) for each recession segment can be computed by linear regression after taking the logarithm of the streamflow: LogðQÞ ¼ c 0 K t where Q is the observed streamflow; t is time [day], and K and c 0 are the linear regression coefficients. For the given time period, in this case , a number of recession events will be identified as samples. Regression will be ð1þ 1of6

2 conducted with equation (1) to compute the K for each of the recession events to enable the probability distribution of the recession slopes to be constructed for different time periods selected for individual case study watersheds. [5] To diagnose the climate and human impacts on the baseflow recession slope trend, Wang and Cai [2009] presented a baseflow recession analysis approach. A brief description of the approach is given below, with some extensions that make the approach more general. The groundwater storage discharge equation is: S ¼ aq b s where Q s (mm d 1 ) is the groundwater discharge; S (mm) is the unconfined groundwater storage; a and b are watershed parameters. The groundwater water balance equation: ds dt ¼ Q s þ Q p þ Q et where Q p (mm d 1 ) is the pumping rate from the aquifer and Q et (mm d 1 ) is the groundwater evaporation. The streamflow equation is: Q ¼ Q s Q r þ ð1 Þ Q p þ Q r þ Q where Q r is the water withdrawal from the river, Q D represents deep groundwater pumping and water diversions from outside of the basin, and b is the water consumption coefficient. As compared to Wang and Cai [2009, equation (11)], this analysis (equation (4)) adds water withdrawal from the river and water transfer to the watershed. In addition, the consumption coefficient (b) now has values in the range of [0, 1] to avoid the negative b coefficients that resulted in Wang and Cai [2009] when external water transfers were not considered explicitly in the analysis. Combining equations (2) (4) and assuming that Q p, Q r, and Q D do not change within two time intervals, if b = 1 (i.e., linear reservoir), then Qtþ ð t ð2þ ð3þ ð4þ Þ ¼ ðqt ðþþ Þexpð t=aþ ð5þ where G = Q et + b(q p + Q r + Q D ) Q D is the total water loss. If b = 0.5, Qtþ ð t Þ ¼ Q e þ Q p þ Q et tan 2 " sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi # Qt ðþ Q e Q p þ Q et arctan t Q p þ Q et a where Q e =(1 b)(q p + Q r + Q D ) Q r. The average recession slope for a given recession event is computed by linear regression through equation (1). 3. Study Area and Data [6] The Salt Creek and Des Plaines watersheds are located in the Chicago metropolitan area; the Kankakee watershed crosses the northern border of Indiana and Illinois; the Embarras watershed is located in eastern central Illinois (Figure S1). 1 The land cover in the Salt Creek watershed is 1 Auxiliary materials are available in the HTML. doi: / 2009GL ð6þ consisted of 27%, 42%, and 77% of urban land in 1956, 1964, and 1996, respectively [Spieker, 1970; Salt Creek Watershed Network, The basics of watershed issues, available at ]. Communities in this watershed relied upon groundwater pumping for the water supply until the 1980 s when they began receiving water diversions from Lake Michigan. By the early 1990 s, the communities became mostly dependent on diversions from Lake Michigan. Meanwhile, effluent discharge has increased with the population growth [Wang and Cai, 2009]. The gage station in Des Plaines is located in the upstream of the Salt Creek (Figure S1), and the urban area of this watershed covers 59% and 66% of the total land in 1990 and 2000, respectively [Friedel, 1998]. [7] The Kankakee River, in which over 75% of the land use was predominantly agriculture in 1998, originates in northwestern Indiana and crosses westward into the northeastern portion of Illinois. Irrigation is widespread due to the lack of water retention of its sandy soils. Much of the irrigation water use in the Indiana part is withdrawn from the Kankakee River and the shallow sand gravel aquifers. According to the estimate of Indiana Department of Natural Resources (IDNR) [1990], water withdrawals in 1987 in the watershed for irrigation, public and domestic water supply, and other uses occupied one third of the total water use, respectively. With population growth and increase of cropland area, the total water use has increased in both winter and summer. In contrast to Kankakee, the Embarras watershed is dominated by rainfed agriculture. According to the water use data from U.S. Geological Survey (USGS) [USGS, 2009] irrigation water use accounted for 0.8% of total water use in [8] Four gage stations from the National Water Information System are used for analysis: Salt Creek at Western Springs ( ), Des Plaines River near Des Plaines ( ), Kankakee River at Momence ( ), and Embarras River at Ste. Marie ( ). The drainage areas for these four sites are 293, 932, 5941, and 3926 km 2, respectively. There is no storage regulation above any of the gage stations in the selected watersheds. Table 1 shows the available time periods of the daily streamflow records. [9] One National Climatic Data Center weather station located in Wheatfield, IN, was selected to support the analysis because it has the longest historical data record (i.e., ) in the region. The Mann Kendall nonparametric test for trends is conducted for the temperature in the winter (December, January, and February) and summer (June, July, and August), and for the total precipitation during winter and summer as well as the annual basis. For temperature, there is a decreasing trend in January, June and July, but no trend exists in February, December and August. There are increasing trends for winter and summer precipitation (See Figures S2, S3, S4, and Table S1). 4. Results and Discussion [10] The analysis is conducted for separated time periods by splitting the available time periods into three sub periods, which results in three 20 year periods for Salt Creek and Des Plaines and three 30 year periods for Kankakee and Embarras (Table 1). 2of6

3 Table 1. Mean, Coefficient of Variation (CV) and Trends of Baseflow Recession Slope Winter Summer Watersheds Time Period Mean CV Trend Mean CV Trend Urban Salt Creek Salt Creek Salt Creek Des Plaines Des Plaines Des Plaines Agriculture Kankakee Kankakee Kankakee Embarras No Embarras No Embarras No 4.1. Climate and Human Impacts on the Baseflow Magnitudes [11] The MK test is conducted to evaluate the trend of low flow magnitude [Sahoo and Smith, 2009]. For Salt Creek and Des Plaines River, an increasing trend exists in winter and summer; while for Kankakee River and Embarras River an increasing trend exists in winter and no statistically significant change in summer. Brutsaert [2008] observed that the Illinois River has experienced an increasing trend with the 7 day minimum flow during the periods between April 15 and October 31 since the 1940 s, and correlated this trend to the increasing groundwater storage. The increasing trend of annual low flows in the Mississippi River and many rivers in Iowa has been attributed to land cover change [Zhang and Schilling, 2006]. During the last century, agricultural development in the Midwest has converted perennial grasslands to annual row crops. Perennial vegetation transpires throughout the spring, summer and fall seasons, while the majority of the transpiration from row crops occurs during the summer. As a result, the annual ET loss has a decreasing trend, which, in turn, increases the baseflow [Schilling et al., 2008]. [12] Compared to the Des Plaines, the increase of baseflow magnitude in Salt Creek is much more significant, especially in summer (Figure 1). This was mainly caused by the water diversion from Lake Michigan and the associated increase of return flow. For Kankakee and Embarras, the increasing trend of low flows in summer is not statistically significant, but a statistically significant increasing trend is found in winter for both agricultural watersheds. This can be caused by land cover change from perennial vegetation to row crops in these two watersheds [IDNR, 1998a], which decreases ET and then increases the low flow in winter. Increasing return flow from the growing urban areas (e.g., small towns) in the two watersheds must also contribute to the increasing trend. Moreover, climate change in the region, i.e., increasing precipitation and the decreasing temperature (See Figure S3, S4 and Table S1 and also refer to IDNR [1998a]), also contributes to the increasing baseflow in winter. Thus both climate and human factors are the causes of the increasing trend of the low flow magnitude in the four watersheds Climate and Human Impacts on the Baseflow Recession Shapes and Slopes [13] Figure 1 shows the constructed MRC [Posavec et al., 2006] in winter and summer during the early and recent time periods (i.e., the second period of MRC is not shown for visual clarity). To quantitatively detect the change of recession slopes, as described in equation (1), the slopes are computed for each of the recession segments longer than three days in a semi log plot. One particular issue is the starting point of baseflow recession and it has been discussed by many previous studies. In our study, the urban watersheds are highly urbanized and have a smaller drainage area. For these watersheds, the starting point of the baseflow recession is set as four days after the peak flow. While for the agricultural watersheds, it is set at eight days after peak flow. Table 1 shows the mean value and the coefficient of variation (CV) of the recession slopes. Student s t test is conducted for the statistical significance of the difference of the slope means between two periods (Table S2 for details). As shown in Table 1, the decreasing (minus sign) and increasing (plus sign) trends are statistically significant except for the summer season in the Embarras River. These trends reflect the impact of both climate and human interferences. Precipitation does not have a direct impact on the recession slopes as shown in equations (5) and (6). Temperature change, land use change, groundwater pumping, surface water withdrawal, and return flow can potentially affect the recession slopes, and the following discussion will diagnose these factors on slope change for each of the four watersheds. [14] In the two urban watersheds, the recession slopes decrease over time in both winter and summer. The MRC for Salt Creek becomes concave from straight line in winter (Figure 1a), and becomes more concave in summer (Figure 1b). The MRC for Des Plaines is a straight line during the early period but becomes slightly concave in winter (Figure 1c) and summer (Figure 1d). During winter when ET is negligible, the temperature change impact on recession slope can be ignored. Referring to the long term solution of the harmonic linearized Bousinessq equation, we have a 1/KD, where a is defined in Equation (2), K is the hydraulic conductivity, and D is groundwater storage [Brutsaert, 2005]. As a consequence, the increase of D (attributed to increasing precipitation) will cause an increasing slope. Thus, neither precipitation nor temperature contributes to the decreasing slope, and we conclude that the decreasing slope in winter must be caused by human interferences, including groundwater pumping, water diversion and effluent discharge. [15] In summer, decreasing temperature and the increasing impervious area in Salt Creek and Des Plaines watersheds reduce ET and thus decrease the recession slope. However, the strong concave shape of MRC in Salt Creek, compared to Des Plaines River, is due to the return flow from water diversion. [16] For the agricultural watersheds, the winter recession slopes decrease for both Kankakee and Embarras and the MRCs are straight lines (during late period of MRC) for both periods (Figure 1e and 1g). The increased groundwater storage (i.e. D), which contributes higher low flow, will cause a larger recession slope [Brutsaert, 2008]. Consequently, the decreasing recession slope is caused by neither the groundwater storage trend nor the increasing precipita- 3of6

4 Figure 1. Master recession curves during winter and summer for Salt Creek, Des Plaines River, Kankakee River, and Embarras River gages during earlier and recent time periods. 4of6

5 Figure 2. The impact of water withdrawals, water consumption rate, and the ratio of groundwater pumping and river water withdrawal to the baseflow recession slope. tion trend in the agricultural watersheds. On the other hand, the total water withdrawal for public supply and domestic use in counties partially located in Embarras increased 70% from 1995 to 2005; in Kankakee the increase is 77% [USGS, 2009]. Thus similar to the two urban watersheds in winter, the important factors are the increasing water withdrawal and return flow from the small towns within the watersheds. [17] The trends for agricultural watersheds in summer are complex. The summer recession slope in Kankakee has increased and the shape of the MRC has changed from a concave curve to a straight line (Figure 1f). Given the deceasing trend of temperature in the region (Figure S3 and Table S1) a decreasing trend of recession slope should appear because of the decreasing ET. Thus temperature is not a factor for the increasing recession slope with Kankakee. The increase of groundwater storage (D) will lead to the decrease of a (equation (2)) and consequently an increase of the recession slope (equations (5) or (6)). However, an increase in water table can not affect the shape of recession curve (e.g., from concave to straight line); for example, for linear reservoirs the recession curve is always exponential regardless the value of a (equation (5)). Thus, the human interferences including groundwater pumping and surface water withdrawal for irrigation must be the principal factors in the change of MRC in summer for the Kankakee River. [18] The shape of summer MRC in Kankakee River becomes straight from concave (Figure 1f) even though the change is not significant and the trends are similar to the findings by Schilling and Helmers [2008] who found that recession in tile drained agricultural watersheds is more linear than less tiled watersheds. Both Kankakee and Embarras River watersheds are tile drained, and the agricultural land area has increased for both watersheds. However, there is no change on the recession slopes and the MRC remains the same concave shape with Embarras (Figure 1h), compared to the shape change (from concave to straight line) with Kankakee. Thus the impact of tile drainage may not dominate the shape change with Kankakee; otherwise why is there no such change with Embarras? We conjecture that the different slope trends between Kankakee and Embarras are actually caused by different agricultural practices, i.e., rainfed agriculture in Embarras and irrigated agriculture in Kankakee. The total farmland area has increased 27% in Embarras since 1925 [IDNR, 1998b], but the increase of rainfed cropland does not change the ET in summer significantly compared to the ET with natural vegetation. This explains the stable slope with Embarras. In Kankakee, the number of irrigated acres has roughly doubled every ten years since 1949 [IDNR, 1998a], and the increasing irrigation water withdrawal leads to higher ET than natural vegetation in summer and thus an increasing recession slope. [19] Another question is why the trend in Kankakee is opposite in winter and summer. This can be explained by the difference of human water use regimes between winter and summer, as reflected by the consumption coefficient which is smaller in winter than in summer. Equation (5) or (6) can be used to understand the impact of b. For a linear reservoir, the decreasing slope indicates a decreasing total water loss, i.e., decreasing water consumption. Since b in winter does not change over years, the water withdrawal decreases. However, water withdrawals, mainly for the domestic and public purpose, increase over years [USGS, 2009]. Thus the linear reservoir equation is not suitable for explaining the winter baseflow states in Kankakee. For nonlinear reservoir, Figures 2a and 2b plot a series of hypothetical recession events during winter and summer, respectively. The groundwater pumping and surface water withdrawal are set to be equal and b is set to 0.1 in winter (Figure 2a) and 0.5 in summer (Figure 2b); the groundwater storage loss due 5of6

6 to ET is set to mm/d in winter and 0.1 mm/d in summer. As shown, with larger total water withdrawals, the recession slope decreases in winter but increases in summer. This provides evidence of the opposite slope trend. [20] The fraction of water sources (groundwater pumping vs. surface water withdrawals) can also affect the recession slope. A hypothetical analysis applied to the Kankakee watershed effectively illustrates that possible effect. As shown in Figures 2c and 2d, assuming a fixed total water withdrawal, the shift of groundwater pumping to the surface water withdrawal increases the recession slope. Thus, the shift of groundwater pumping to surface water withdrawal in summer and the shift of surface water withdrawal to groundwater pumping in winter over the three time periods are the potential reasons for those trends. Although we do not have long term time series data of water withdrawals to show such a shift in the Kankakee watershed, Figure S5 does show an increase of surface water withdrawal fraction in the major part of the watershed in Indiana. 5. Conclusions [21] This paper explores the trends of baseflow magnitudes and recession shapes and slopes in winter and summer in two urban and two agricultural watersheds in the U.S. Midwest. The causing factors for changes in baseflow recession slopes are different from those for the baseflow magnitude. The trend of baseflow recession is mainly caused by human interferences. In the urban watersheds, the decreasing trend of the recession slope in winter and summer is caused by the effluent discharges, groundwater pumping, and water diversions. In the Embarras watershed with rainfed crops, the decreasing recession slope is mainly caused by the effluent discharge from the towns in the watershed and there is no slope change in summer. In the Kankakee Watershed with irrigation, the opposite trends during winter and summer can be explained by seasonal water use regimes and the sources of water withdrawal. This long term analysis of recession rates, in conjunction with the changes in flow magnitude, offers additional insights on human interferences to hydrologic processes. [22] Acknowledgments. The authors are grateful for the helpful suggestions from two anonymous reviewers and the editors. This work was funded by U.S. National Science Foundation (NSF) grants CBET and EAR References Brutsaert, W. (2005), Hydrology: An Introduction, Cambridge Univ. Press, Cambridge, U. K. Brutsaert, W. (2008), Long term groundwater storage trends estimated from streamflow records: Climatic perspective, Water Resour. Res., 44, W02409, doi: /2007wr Friedel, M. J. (1998), National Water Quality Assessment Program, Upper Illinois River Basin, USGS Fact Sheet FS , Water Resour. Div., U.S. Geol. Surv., Urbana, Ill. (Available at uirb/pubs/reports/fs /FS pdf) Illinois Department of Natural Resources (IDNR) (1998a), Kankakee River Area Assessment, vol. 2, Water Resources, Ill. State Water Surv., Champaign, Ill. Illinois Department of Natural Resources (IDNR) (1998b), The Embarras River Basin An inventory of the regions resources, report, Ill. State Water Surv., Champaign, Ill. (Available at pfc/assessments/erp/pagei.htm) Indiana Department of Natural Resources (IDNR) (1990), Water resources availability in the Kankakee River Basin, Indiana Executive summary, Water Res. Assess. 90 3, Division of Water, Indianapolis, Indiana. (Available at Posavec, K., A. Baĉani, and Z. Nakiĉ (2006), A visual basic spreadsheet macro for recession curve analysis, Ground Water, 44(5), , doi: /j x. Sahoo, D., and P. K. Smith (2009), Hydroclimatic trend detection in a rapidly urbanizing semi arid and coastal river basin, J. Hydrol., 367, , doi: /j.jhydrol Schilling, K. E., and M. Helmers (2008), Effects of subsurface drainage tiles on streamflow in Iowa agricultural watersheds: exploratory hydrograph analysis, Hydrol. Processes, 22, , doi: / hyp Schilling, K. E., M. K. Jha, Y. K. Zhang, P. W. Gassman, and C. F. Wolter (2008), Impact of land use and land cover change on the water balance of a large agricultural watershed: Historical effects and future directions, Water Resour. Res., 44, W00A09, doi: /2007wr Spieker, A. M. (1970), Water in urban planning, Salt Creek Basin, Illinois, U.S. Geol. Surv. Water Supply Pap., 2002, 147 p. St. Jacques, J. M., and D. J. Sauchyn (2009), Increasing winter baseflow and mean annual streamflow from possible permafrost thawing in the Northwest Territories, Canada, Geophys. Res. Lett., 36, L01401, doi: /2008gl U.S. Geological Survey (USGS) (2009), Estimated use of water in the united states in 2005, U.S. Geol. Surv. Circ., 1344, 60 pp. (Available at pubs.usgs.gov/circ/1344/pdf/c1344.pdf) Wang, D., and X. Cai (2009), Detecting human interferences to low flows through base flow recession analysis, Water Resour. Res., 45, W07426, doi: /2009wr Zhang, Y. K., and K. E. Schilling (2006), Increasing streamflow and baseflow in Mississippi River since the 1940s: Effect of land use change, J. Hydrol., 324, , doi: /j.jhydrol X. Cai and D. Wang, Department of Civil and Environmental Engineering, University of Illinois at Urbana Champaign, Urbana, IL 61801, USA. (xmcai@illinois.edu) 6of6