Introduction 1995 1998 2002 2006 2000 2000 1997 1995 1997 2001 1988 2001 1997 1988 1991 2000 1997 1997 1997 1997 2000 2001 1997 2001 1995 1994 2002 Methods 2 2 1998a 1 1998b Fig. 1 Location of study watersheds in Kansas, grouped by level III U.S. EPA ecoregion, and example of land cover classification scheme, in which riparian and catchment land cover was quantified for the subcatchment of each stream segment in the watersheds  Twenty-four watersheds were located in the Flint Hills (FH) ecoregion, characterized by rolling hills, coarse soils, and relatively intact tracts of tallgrass prairie predominantly used as cattle pasture. Because of topography and geology, little of this region has been converted to cropland agriculture. Eighteen watersheds were located in the Central Irregular Plains (CIP), characterized by irregular topography, loam soils, and a variety of land use types, including cropland agriculture, tallgrass prairie, and oak-hickory forests. Fourteen watersheds were located in the Western Corn Belt Plains (WCBP), a region that was historically covered with tall and mixed-grass prairie but has now been almost entirely converted to cropland agriculture. Finally, 12 watersheds were located in the eastern part of the Central Great Plains (CGP) ecoregion, characterized by reduced topography, mixed-grass prairie, and large tracts of cropland agriculture. Criteria for inclusion in the study were as follows: (1) watersheds were sampled for water chemistry parameters a minimum of 12 times, and (2) watersheds were entirely contained within one U.S. EPA level III ecoregion. Watersheds were located across a precipitation gradient, with average rainfall ranging from 610 to 1016 mm/year. No watersheds were chosen that had very large livestock feeding operations or municipal point sources. The few smaller feeding operations (∼1000 animals) included were in all cases at least 0.1 km upstream of the stream chemistry site, and the total area of these operations was included in the analysis (see section Statistical Analyses, below). 2 2000 2001 Fig. 2 A B C  1957 Water Chemistry Data 2000 3 − 4 + 4 + 3 − 1983 4 + 1983 1992 3 − 4 + 3 − 4 + 3 − 4 + Digital and Land Cover Data 2002 1 2001 1976 1993 Statistical Analyses F r 2004 Results Riparian-Water Chemistry Relationships 1 3 3 − R 2 R 2 Table 1 Multiple regression models showing correlations between water chemistry parameters and riparian land cover in both the whole watersheds the first-order streams of watersheds Water chemistry parameter Crop Forest Grassland Urban  Intercept R 2 Watershed   TN   −0.440 0.260 1.932  0.355 3 0.623   0.490 −0.500 0.525 4  −0.466 −0.662  0.203 0.327   TP 0.264   0.712 0.095 0.507   AT 0.428    0.558 0.171   FC 0.378    1621.570 0.199   DO (max) 0.508    12.085 0.247 First order   TN 0.388   0.576 0.551 0.406 3 0.650   0.538 −0.033 0.606 4  −0.445 −0.683  0.195 0.304   TP 0.320   0.780 0.087 0.634   AT 0.413    0.605 0.158   FC 0.458    798.832 0.198   DO (max) 0.522    12.113 0.261 Note p  Fig. 3 R 2 A B R 2 p   Riparian land cover 2 and 4 km upstream explained no significant variance in TP concentrations, and riparian land cover 2 km upstream of the sampling point explained no significant variance in AT concentrations. Total suspended solids and minimum DO concentrations did not have significant relationships with riparian cover in any analyses and are not discussed further in this section. 3 2 Table 2 Partial correlations among nutrient concentrations and riparian land cover classifications Water chemistry parameter Catchment land cover  Riparian land cover r p Watershed   TN Grass, forest Grass, urban Grass = −0.06 0.687 Urban = 0.20 0.134 3 − Crop, urban Crop, urban Crop = 0.22 0.071 Urban = 0.48 0.000 4 + Grass, forest Grass, forest Grass = −0.03 0.803 Wood = −0.11 0.370   TP Crop, urban, forest Crop, urban Crop = 0.04 0.779 Urban = 0.58 0.000 First order   TN Crop, grass Crop, urban Crop = 0.25 0.068 Urban = 0.33 0.013 3 − Crop, urban Crop, urban Crop = 0.26 0.033 Urban = 0.50 0.000 4 + Grass, forest Grass, forest Grass = −0.00 0.994 Forest = −0.08 0.543   TP Crop, urban Crop, urban Crop = 0.04 0.724 Urban = 0.68 0.000 Note r 1 3 Temporal Variation Examination of regional discharge patterns revealed that 25% of annual water volume was discharged from January to April, 50% by June, 75% by August, and the remainder in the August–December time period. Thus, the periods of January–April, May, June–July, and August–December were designated as seasons in temporal analyses. Seasons in which a quarter of annual water volume was discharged in 1 or 2 months (i.e., May, June–July) represented periods of high flow and high connectivity across the landscape, while seasons encompassing more than 2 months (January–April, August–December) represented predominantly base flow conditions (with most of the upper reaches of the first-order streams dry). 4 + 4 3 − Fig. 4 R 2 A B R 2 p   R 2 3 − Impact of Stream Size 3 − 5 Fig. 5 R 2  Ecoregion Effects 3 − 4 + 3 − p  6 4 + Fig. 6 Mean values for selected water chemistry parameters and riparian cropland, grouped by ecoregion (WCBP, Western Corn Belt Plains; CGP, Central Great Plains; FH, Flint Hills; CIP, Central Irregular Plains). TN data for WCBP were available for only 3 of 12 study watersheds. Significant differences are labeled with different letters; error bars represent 1 SE  The Flint Hills was the only ecoregion that exhibited significantly different TP concentrations, which were lower than those of other ecoregions. Atrazine concentrations did not differ significantly among ecoregions. 7 3 Fig. 7 3 −  Table 3 Comparisons of least-squares means using general linear model analyses to assess differences in slopes of riparian-water chemistry relationships at four spatial scales, across level III U.S. EPA ecoregions Spatial scale Response variable Ecoregions with different slopes p Watershed TP FH & CGP 0.010 First order TN FH & CGP 0.019 First order TP FH & CGP 0.007 First order TP CIP & CGP 0.007 First order TP CIP & WCBP 0.041 2 km upstream TN FH & CGP 0.022 2 km upstream 3 − FH & CIP 0.014 2 km upstream 3 − FH & WCBP 0.002 4 km upstream 3 − FH & WCBP 0.001 4 km upstream FC FH & WCBP 0.023 Note p  Discussion Land Cover-Water Chemistry Relationships 1997 3 − 2007 3 − 3 − 4 + 3 − 4 + 2001 4 + 3 −  2000 4 + 1997 4 + 1994 5 4 + 3 − 1996 3 − 2000 2003 3 − 3 −  1997 2001 1997 2001 1988 2001 1945 1964 3 − 1997 2000 2000 2001 2000 2001 2007 2001 1995 2001 1988 1997 3 − 3 − 1978 1997 1978 1984 1997 1988 Ecoregion Effects 1998a 1998b 1995 1995 1997 1988 Conclusions The data suggest that riparian cover near sampling sites is generally less well correlated with water quality parameters than riparian cover or land use in first-order streams. Because watershed cover and riparian land use were correlated, it is difficult to determine how important first-order riparian cover is related to water quality. Our results suggest a statistically significant effect of riparian cover of first-order streams on water quality because partial correlations among riparian land cover classifications were significant predictors in regression models when controlling for predictor catchment land cover classifications. We take the conservative approach in our interpretation, but it is possible that riparian cover has much stronger effects than whole-watershed land cover and that most of the correlation is driven by riparian effects. The effect of first-order land cover may not be too surprising; first-order streams make up the majority of stream length in watersheds. Our approach shows that a correlation with land uses in small headwater streams does hold, and holds even in seasons when many of the first-order stream channels are not flowing.