USGS - science for a changing world

Kansas Water Science Center

U.S. Geological Survey
Water-Resources Investigations Report 99-4242
Prepared in cooperation with the
KANSAS DEPARTMENT OF HEALTH AND ENVIRONMENT

Estimation of Potential Runoff-Contributing Areas in Kansas Using Topographic and Soil Information

By Kyle E. Juracek

CONTENTS

FIGURES

    Figure 1. Map showing location of major river basins and selected subbasins in Kansas
    Figure 2. Schematic diagrams illustrating infiltration-excess overland flow and saturation-excess overland flow
    3-4. Maps showing:
      Figure 3. Soil permeability in Kansas
      Figure 4. Topographic wetness index data for Kansas
    5-10. Maps showing potential contributing and noncontributing areas of combined infiltration- and saturation-excess overland flows in Kansas for:
      Figure 5. Very high potential-runoff conditions
      Figure 6. High potential-runoff conditions
      Figure 7. Moderate potential-runoff conditions
      Figure 8. Low potential-runoff conditions
      Figure 9. Very low potential-runoff conditions
      Figure 10. Extremely low potential-runoff conditions

TABLE

    Table 1. Potential contributing areas for combined infiltration- and saturation-excess overland flows for selected subbasins in Kansas

CONVERSION FACTORS, ABBREVIATIONS, AND VERTICAL DATUM

Multiply By To obtain
inch (in.) 2.54 centimeter
inch per hour (in/hr) 2.54 centimeter per hour
kilometer(km) 0.6214 mile
meter (m) 3.281 foot
mile (mi) 1.609 kilometer
square kilometer (km²) 0.3861 square mile
square mile (mi²) 2.590 square kilometer

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929-a geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea Level Datum of 1929.


Estimation of Potential Runoff-Contributing Areas in Kansas Using Topographic and Soil Information

By Kyle E. Juracek

Abstract

Digital topographic and soil information was used to estimate potential runoff-contributing areas throughout Kansas. The results then were used to compare 91 selected subbasins representing soil, slope, and runoff variability. Potential runoff-contributing areas were estimated collectively for the processes of infiltration-excess and saturation-excess overland flow using a set of environmental conditions that represented very high, high, moderate, low, very low, and extremely low potential runoff. For infiltration-excess overland flow, various rainfall-intensity and soil-permeability values were used. For saturation-excess overland flow, antecedent soil-moisture conditions and a topographic wetness index were used.

Results indicated that very low potential-runoff conditions provided the best ability to distinguish the 91 selected subbasins as having relatively high or low potential runoff. The majority of the subbasins with relatively high potential runoff are located in the eastern half of the State where soil permeability generally is less and precipitation typically is greater. The ability to distinguish the subbasins as having relatively high or low potential runoff was possible mostly due to the variability of soil permeability across the State.

INTRODUCTION

The State of Kansas is required by the Federal Clean Water Act of 1972 to develop a total maximum daily load (TMDL) for basins throughout the State. A TMDL is an estimate of the maximum pollutant load (material transported during a specified time period) from point and nonpoint sources that a receiving water can accept without exceeding water-quality standards (U.S. Environmental Protection Agency, 1991). Requisite for the development of TMDL's is an understanding of potential source areas of storm runoff that are the most likely contributors of nonpoint-source pollution within a basin.

A study by the U.S. Geological Survey (USGS), in cooperation with the Kansas Department of Health and Environment, was begun in 1999 to estimate the spatial extent and pattern of potential runoff-contributing areas in Kansas (fig. 1). The specific study objectives were to:

    (1) Estimate potential runoff-contributing areas resulting from infiltration-excess and saturation-excess overland flows; and
    (2) Compare potential runoff among major river basins and selected subbasins throughout the State.

The purpose of this report is to present the results of the study to estimate the spatial extent and pattern of potential runoff-contributing areas in Kansas. The methods presented in this report may be applicable nationwide as related to the development of TMDL's and the targeting and implementation of best-management practices (BMP's). This study is a statewide expansion of a recently completed study by Juracek (1999) in which the same type of data and similar methods were used to estimate potential runoff-contributing areas in the Kansas-Lower Republican River Basin in Kansas. This study was made possible in part by support from the Kansas State Water Plan Fund.

Background

Runoff-contributing areas within river basins primarily are the result of two processes, both of which produce overland flow. The first process is infiltration-excess overland flow (fig. 2A), which occurs when precipitation intensity exceeds the rate of water infiltration into the soil. This process may be dominant in basins where the land surface has been disturbed (for example, plowed cropland) or where natural vegetation is sparse. The second process is saturation-excess overland flow (fig. 2B), which occurs when precipitation falls on temporarily or permanently saturated land-surface areas that have developed from "outcrops" of the water table at the land surface (Hornberger and others, 1998). A temporary water table can develop during a storm when antecedent soil-moisture conditions in a basin are high. The saturated areas where saturation-excess overland flow develops expand during a storm and shrink during extended dry periods (Dunne and others, 1975).

Historically, infiltration-excess overland flow has been assumed to be the most important runoff process in Midwestern agricultural areas. More recently, saturation-excess overland flow has been considered an important runoff process and is the subject of ongoing research.

Both runoff processes would be expected to affect the load of water-quality constituents in streams, although possibly in different ways due to different flow paths. The identification of potential runoff-contributing areas in a basin can provide guidance for the targeting of BMP's to reduce runoff and meet TMDL requirements. Implementation of BMP's within potential runoff-contributing areas is likely to be more effective at reducing constituent loads compared to areas less likely to contribute runoff.

The spatial extent and pattern of runoff-contributing areas are affected by climate, soil, and terrain characteristics. Contributing areas of infiltration-excess overland flow are determined by the combined effect of rainfall intensity and soil permeability. The least-permeable soils in a basin are the most likely to contribute infiltration-excess overland flow. As rainfall intensity increases, areas with more moderate permeability also may contribute overland flow.

Contributing areas of saturation-excess overland flow are determined by the combined effect of basin topography and antecedent soil-moisture conditions. The effect of topography on saturation-excess overland flow can be quantified by an index called the topographic wetness index (TWI) (Wolock and McCabe, 1995). The TWI is computed as ln (a/S) for all points in a basin, where "ln" is the natural logarithm, "a" is the upslope area per unit contour length, and "S" is the slope at that point. The locations in a basin with the highest TWI values (large upslope areas and gentle slopes) are the most likely to contribute saturation-excess overland flow. When antecedent soil-moisture conditions are dry, only areas with the highest TWI values may be saturated and potentially contribute overland flow. When antecedent soil-moisture conditions are wet, areas with lower TWI values may be saturated and potentially contribute overland flow.

Description of Kansas

Kansas encompasses an area of about 82,000 mi². Major river basins in Kansas include the Cimarron, Kansas-Lower Republican, Lower Arkansas, Marais des Cygnes, Missouri, Neosho, Smoky Hill-Saline, Solomon, Upper Arkansas, Upper Republican, Verdigris, and Walnut (fig. 1). Numerous Federal reservoirs are located throughout the eastern two-thirds of the State. Land use is predominantly agricultural, with cropland, grassland, and woodland accounting for 53.0, 42.7, and 2.5 percent of the State, respectively. Urban land use accounts for about 1 percent of the State (Kansas Applied Remote Sensing Program, 1993).

Terrain varies throughout Kansas and includes flat plains, rolling hills, sandhills, and steep slopes (Moody and others, 1986). Soil permeability ranges from about 0.3 to 12.5 in/hr (fig. 3), with a mean of about 1.6 in/hr. The highest soil-permeability values occur in the Cimarron and Upper and Lower Arkansas River Basins of southwest and south-central Kansas. Soil permeability also is generally higher in the western half of the State. Across the State, soil permeability is typically higher in the flood plains of the major rivers and streams (U.S. Department of Agriculture, 1993). Mean annual precipitation ranges from about 15 in. or less in extreme western Kansas to about 40 in. in the southeast (Paulson and others, 1991).

ESTIMATION OF POTENTIAL RUNOFF-CONTRIBUTING AREAS

Within the State, 91 subbasins representing soil, slope, and runoff variability were selected for analysis (fig. 1). The selected subbasin boundaries were obtained from a statewide data base of 11- and 14-digit hydrologic unit (basin) boundaries that was developed at a scale of 1:24,000 (U.S. Department of Agriculture, Natural Resources Conservation Service, 1997). For all selected subbasins, potential runoff-contributing areas were estimated collectively for the processes of infiltration-excess and saturation-excess overland flow. Geographic-information-system (GIS) techniques and available digital data were used to perform the spatial analyses required to estimate potential runoff-contributing areas. All analyses were done using the GRID module of the ArcInfo GIS software package. The digital data used in the analyses were in a grid (raster) format with a grid-cell size of 1 km². The digital data included the U.S. Department of Agriculture's 1:250,000-scale State soils geographic data base (STATSGO) (U.S. Department of Agriculture, 1993) and the USGS 1-km-resolution digital elevation model (DEM) (Verdin and Greenlee, 1996). These two digital data sets are suitable for comparing potential runoff among basins hundreds of square miles in size. Thus, in this study emphasis was placed on a comparison of potential runoff-contributing areas between, rather than within, individual subbasins.

The potential for infiltration-excess overland flow was estimated using STATSGO-derived soil-permeability digital data. In the STATSGO data set, soil permeability represents the infiltration rate when the soil is saturated (Soil Survey Staff, 1997). In general, there is an inverse relation between soil permeability and the potential for infiltration-excess overland flow. Using GIS techniques and digital maps of State and subbasin boundaries, the soil-permeability data were extracted from the STATSGO data base for the State and each subbasin (fig. 3). The mean soil permeability then was computed for each subbasin.

An equal-interval approach was used to select six threshold soil-permeability values that represent the rainfall intensity at which infiltration-excess overland flow would occur. In Kansas, soil permeability ranges from about 0.3 to 12.5 in/hr. However, because about 93 percent of the State has a soil permeability of 4.0 in/hr or less, the effective range used in this study was 0.3 to 4.0 in/hr. Thus, the threshold soil-permeability values, representing very high, high, moderate, low, very low, and extremely low rainfall intensity, were set at 3.38, 2.76, 2.14, 1.52, 0.90, and 0.45 in/hr, respectively. The extremely low threshold value was adjusted upward from 0.28 to 0.45 in/hr because the original value was less than the range of soil-permeability values for the State. In general, the lower rainfall intensities occur more frequently than the higher rainfall intensities. The higher soil-permeability thresholds imply a more intense storm during which areas with higher soil permeability potentially may contribute infiltration-excess overland flow. The threshold soil-permeability values were used to compare the selected subbasins on the basis of the percentage of each subbasin with soil-permeability values that were less than or equal to the threshold value and thus potentially contribute infiltration-excess overland flow.

The potential for saturation-excess overland flow was estimated using DEM-derived TWI digital data. In general, there is a direct relation between TWI and the potential for saturation-excess overland flow. Derivation of the TWI digital data followed the approach described by Wolock and McCabe (1995). Elevation differences among the grid cells in the DEM were compared and used to create a flow-direction grid (Jenson and Domingue, 1988). The flow-direction grid was used to derive a flow-accumulation grid by computing the number of upslope cells that drain into each cell. The upslope area per unit contour length (a) for each cell in the flow-accumulation grid was computed as:

(1)           a =   (number of upslope cells + 0.5) x (grid-cell length).

Using the DEM and the flow-direction grid, the magnitude of the slope (S) was computed for each cell as:

     (2)            S =   (change in elevation between neighboring grid cells) /
(horizontal distance between centers of neighboring grid cells).

The resultant slope (gradient) grid then was used in combination with the flow-accumulation grid to compute TWI for each cell as:

(3)            TWI =    ln (a /S).                                                     

Using GIS techniques and digital maps of State and subbasin boundaries, the TWI grid data were extracted for the State and each subbasin (fig. 4). The mean TWI was computed for each subbasin. An equal-interval approach was used to select six threshold TWI values that represented a range of wet-to-dry antecedent soil-moisture conditions. In Kansas, the TWI ranges from 8.6 to 28.7. However, because about 95 percent of the State has a TWI of 19 or less, the effective range used in this study was 8.6 to 19. Thus, the threshold TWI values, representing very wet, wet, moderate, dry, very dry, and extremely dry antecedent soil-moisture conditions, were set at 10.4, 12.1, 13.8, 15.6, 17.3, and 19.0, respectively. The lower TWI thresholds imply wetter antecedent soil-moisture conditions during which areas with lower TWI values potentially may contribute saturation-excess overland flow. The threshold TWI values were used to compare the selected subbasins on the basis of the percentage of each subbasin that had TWI values greater than or equal to the threshold value and thus potentially contribute saturation-excess overland flow.

The combined potential for runoff in Kansas and the selected subbasins due to infiltration-excess and saturation-excess overland flows was estimated by merging the previously described hypothetical conditions. A very high potential-runoff condition was created by combining very high rainfall intensity (soil permeability less than or equal to 3.38 in/hr) with very wet antecedent soil-moisture (TWI greater than or equal to 10.4) conditions. A high potential-runoff condition was created by combining high rainfall intensity (soil permeability less than or equal to 2.76 in/hr) with wet antecedent soil-moisture (TWI greater than or equal to 12.1) conditions. A moderate potential-runoff condition was created by combining moderate rainfall intensity (soil permeability less than or equal to 2.14 in/hr) with moderate antecedent soil-moisture (TWI greater than or equal to 13.8) conditions. A low potential-runoff condition was created by combining the low rainfall intensity (soil permeability less than or equal to 1.52 in/hr) with dry antecedent soil-moisture (TWI greater than or equal to 15.6) conditions. A very low potential-runoff condition was created by combining the very low rainfall intensity (soil permeability less than or equal to 0.90 in/hr) with very dry antecedent soil-moisture (TWI greater than or equal to 17.3) conditions. An extremely low potential-runoff condition was created by combining the extremely low rainfall intensity (soil permeability less than or equal to 0.45 in/hr) with extremely dry antecedent soil-moisture (TWI greater than or equal to 19.0) conditions. The combined conditions were used to compare the selected subbasins on the basis of the percentage of each subbasin that potentially contributes runoff by one or both overland-flow processes.

POTENTIAL RUNOFF-CONTRIBUTING AREAS

For very high potential-runoff conditions (soil permeability less than or equal to 3.38 in/hr, TWI greater than or equal to 10.4), 89 of 91 (98 percent) subbasins had potential contributing areas in greater than 90 percent of each subbasin (table 1). Thus, this set of environmental conditions was not useful for the purpose of distinguishing subbasins as having relatively high or low potential runoff. The spatial extent and pattern of combined potential contributing and noncontributing areas for very high potential-runoff conditions are shown in figure 5.

For high potential-runoff conditions (soil permeability less than or equal to 2.76 in/hr, TWI greater than or equal to 12.1), 85 of 91 (93 percent) subbasins had potential contributing areas in greater than 90 percent of each subbasin (table 1). Thus, with two exceptions, this set of environmental conditions was not useful for the purpose of distinguishing subbasins as having relatively high or low potential runoff. The two exceptions were within the Cimarron and Lower Arkansas River Basins (fig. 1). In the Cimarron Basin, Cavalry Creek (subbasin 1, 73.8 percent) had substantially less potential contributing area than Crooked Creek (subbasin 2, 96.4 percent). In the Lower Arkansas Basin, potential contributing areas ranged from 71.6 percent of the subbasin for Mule Creek (subbasin 28) to 100 percent for Sun and Turkey Creeks (subbasin 33). Of the 12 subbasins in the Lower Arkansas Basin, 5 (42 percent) had potential contributing areas between 70 and 90 percent. The spatial extent and pattern of combined potential contributing and noncontributing areas for high potential-runoff conditions are shown in figure 6.

Potential contributing areas were greater than 90 percent for 82 of 91 (90 percent) subbasins for moderate potential-runoff conditions (soil permeability less than or equal to 2.14 in/hr, TWI greater than or equal to 13.8). The ability to distinguish subbasins as having relatively high or low potential runoff again was restricted to the Cimarron and Lower Arkansas River Basins. In the Cimarron Basin, Cavalry Creek (subbasin 1, 54.9 percent) again had substantially less potential contributing area than Crooked Creek (subbasin 2, 86.9 percent). In the Lower Arkansas Basin, potential contributing areas ranged from 63 percent of the subbasin for Mule Creek (subbasin 28) to 100 percent for Sun and Turkey Creeks (subbasin 33). Of the 12 subbasins in the Lower Arkansas Basin, 5 (42 percent) had potential contributing areas between 70 and 90 percent, and 2 (17 percent) had potential contributing areas less than 70 percent (table 1). In the remaining 10 major river basins, all subbasins had potential contributing areas greater than 90 percent. The spatial extent and pattern of combined potential contributing and noncontributing areas for moderate potential runoff conditions are shown in figure 7.

For low potential-runoff conditions (soil permeability less than or equal to 1.52 in/hr, TWI greater than or equal to 15.6), 75 of 91 (82 percent) subbasins had potential contributing areas in greater than 90 percent of each subbasin (table 1). Therefore, the ability to distinguish subbasins as having relatively high or low potential runoff was still limited. Again, the best ability to distinguish subbasins was observed in the Cimarron and Lower Arkansas River Basins. In the Cimarron Basin, the difference in potential contributing area between Cavalry Creek (subbasin 1, 44.2 percent) and Crooked Creek (subbasin 2, 82.9 percent) remained substantial. In the Lower Arkansas Basin, potential contributing areas ranged from 24.1 percent of the subbasin for the North Fork Ninnescah River upstream from Cheney Reservoir (subbasin 29) to 100 percent for Sun and Turkey Creeks (subbasin 33). Of the 12 subbasins in the Lower Arkansas Basin, 4 (33 percent) had potential contributing areas of between 70 and 90 percent, and 5 (42 percent) had potential contributing areas of between 20 and 60 percent.

Across the State, subbasins with potential contributing areas less than 90 percent were observed in three other major basins for low potential-runoff conditions. In the Kansas-Lower Republican River Basin, the Republican River upstream from Concordia (subbasin 14) had a potential contributing area of 85.3 percent of the subbasin. In the Smoky Hill-Saline River Basin, the Saline River upstream from Wilson Lake (subbasin 61) and the Smoky Hill River upstream from Cedar Bluff Reservoir (subbasin 63) had respective potential contributing areas of 82.1 and 77.9 percent. Finally, in the Solomon River Basin, Bow Creek (subbasin 66) and the South Fork Solomon River upstream from Webster Reservoir (subbasin 74) had respective potential contributing areas of 79.5 and 76.1 percent. The spatial extent and pattern of combined potential contributing and noncontributing areas for low potential-runoff conditions are shown in figure 8.

Overall, the best statewide ability to distinguish subbasins was observed for the very low potential-runoff conditions (soil permeability less than or equal to 0.90 in/hr, TWI greater than or equal to 17.3). Of the 91 subbasins, 32 (35 percent) had potential contributing areas in greater than 90 percent of each subbasin, 14 (15 percent) had potential contributing areas between 70 and 90 percent, 5 (5 percent) had potential contributing areas between 50 and 70 percent, 7 (8 percent) had potential contributing areas between 30 and 50 percent, 16 (18 percent) had potential contributing areas between 10 and 30 percent, and 17 (19 percent) had potential contributing areas less than 10 percent (table 1). The spatial extent and pattern of combined potential contributing and noncontributing areas for very low potential-runoff conditions are shown in figure 9.

For extremely low potential-runoff conditions (soil permeability less than or equal to 0.45, TWI greater than or equal to 19.0), the ability to distinguish subbasins varied by major river basin. Of the 91 subbasins statewide, 4 (4 percent) had potential contributing areas greater than 90 percent of each subbasin, 7 (8 percent) had potential contributing areas between 70 and 90 percent, 2 (2 percent) had potential contributing areas between 50 and 70 percent, 12 (13 percent) had potential contributing areas between 30 and 50 percent, 9 (10 percent) had potential contributing areas between 10 and 30 percent, and 57 (63 percent) had potential contributing areas less than 10 percent (table 1). The spatial extent and pattern of combined potential contributing and noncontributing areas for extremely low potential-runoff conditions are shown in figure 10.

Using very low potential-runoff conditions, the major river basins were categorized as having either relatively high or relatively low potential runoff. The very low potential-runoff conditions are meaningful because they provide the best ability to distinguish subbasins and because the 0.90-in/hr rainfall intensity occurs more frequently than the higher rainfall intensities. A basin was categorized as having relatively high potential runoff if the majority of its subbasins (those selected for analysis) had potential contributing areas greater than 70 percent of each subbasin. A basin was categorized as having relatively low potential runoff if the majority of its subbasins (those selected for analysis) had potential contributing areas less than 30 percent of each subbasin. The major basins having relatively high potential runoff were the Kansas-Lower Republican (15 of 19 subbasins had potential contributing areas greater than 70 percent of each subbasin), Marais des Cygnes (7 of 8 subbasins), Missouri (4 of 5 subbasins), Neosho (6 of 7 subbasins), Verdigris (4 of 7 subbasins), and Walnut (4 of 4 subbasins). The major basins having relatively low potential runoff were the Cimarron (1 of 2 subbasins had potential contributing areas less than 30 percent of each subbasin), Lower Arkansas (7 of 12 subbasins), Smoky Hill-Saline (9 of 11 subbasins), Solomon (8 of 10 subbasins), Upper Arkansas (2 of 3 subbasins), and Upper Republican (3 of 3 subbasins).

SUMMARY AND CONCLUSIONS

Digital topographic and soil information was used to estimate and compare potential runoff-contributing areas for 91 selected subbasins in Kansas. Potential contributing areas were estimated collectively for the processes of infiltration-excess and saturation-excess overland flow using a set of environmental conditions that represented very high, high, moderate, low, very low, and extremely low potential runoff. For infiltration-excess overland flow, various rainfall-intensity and soil-permeability values were used. For saturation-excess overland flow, antecedent soil-moisture conditions and a topographic wetness index were used.

Results indicated that nearly all subbasins had large percentages of potential runoff-contributing area for the low to very high potential-runoff conditions. Thus, the ability to distinguish subbasins as having relatively high or low potential runoff for those conditions was very limited. The best statewide ability to distinguish subbasins as having relatively high or low potential runoff was provided by the very low potential-runoff conditions. Within the major river basins, the ability to distinguish subbasins as having relatively high or low potential runoff varied. For the Smoky Hill-Saline and Upper Arkansas River Basins, the very low potential-runoff conditions provided the best ability to distinguish subbasins. For the Kansas-Lower Republican, Missouri, Neosho, and Verdigris River Basins, good ability to distinguish subbasins was provided by both the very low and extremely low potential-runoff conditions. For the Lower Arkansas and Solomon River Basins, the best ability to distinguish subbasins was provided by the low and very low potential-runoff conditions. The extremely low potential-runoff conditions provided the best ability to distinguish subbasins for the Marais des Cygnes and Walnut River Basins. In the Cimarron River Basin, good ability to distinguish subbasins was provided by the high, moderate, low, and very low potential-runoff conditions.

The major river basins having relatively high potential runoff were the Kansas-Lower Republican, Marais des Cygnes, Missouri, Neosho, Verdigris, and Walnut. These basins are located in eastern Kansas where soil permeability generally is less and precipitation typically is greater. The major river basins having relatively low potential runoff were the Cimarron, Lower Arkansas, Smoky Hill-Saline, Solomon, Upper Arkansas, and Upper Republican. These basins are located in western Kansas where soil permeability generally is higher and precipitation typically is less.

The ability to distinguish the subbasins as having relatively high or low potential runoff was possible mostly due to the variability of soil permeability across the State. Due to this variability, the subbasins had a wide range of potential contributing areas for infiltration-excess overland flow, particularly for very low potential-runoff conditions. In contrast, the topographic wetness index had a relatively uniform distribution across the State. Thus, the subbasins had a narrow range of potential contributing areas for saturation-excess overland flow. The results presented in Juracek (1999) provide an example of the relative importance of the two overland flow processes for estimating and comparing potential contributing areas within a basin.

Under low potential-runoff conditions characterized by low antecedent soil-moisture content and low rainfall intensity, potentially contributing areas for infiltration-excess and saturation-excess overland flows are limited to areas of lower soil permeability and saturated areas adjacent to rivers and streams, respectively (fig. 10). As antecedent soil-moisture content and rainfall intensity increase, the potential contributing areas for both infiltration-excess and saturation-excess overland flow processes increase. Under high potential-runoff conditions characterized by high antecedent soil-moisture content and high rainfall intensity, the distinction between infiltration-excess and saturation-excess overland flow becomes less meaningful as the ground becomes increasingly saturated and the potential contributing areas for both runoff processes coalesce (fig. 5).

This study had some limitations. First, the digital data sets used were only suitable for use in a comparison of areas hundreds of square miles in size. Thus, the analysis emphasized a comparison of potential runoff-contributing areas between, rather than within, individual subbasins. Improved results will be possible with more spatially detailed digital topographic and soil data sets. Such data sets are currently (1999) being developed and will include the U.S. Department of Agriculture's 1:24,000-scale soil survey geographic data base (SSURGO) and the USGS 30-m-resolution DEM. When available, these data sets will enable a comparison of potential contributing areas for areas tens of square miles in size. Then, a comparison of areas within individual subbasins will be possible. As a result, the spatial extent and pattern of potential contributing areas for infiltration- and saturation-excess overland flows will provide guidance for the implementation of BMP's within individual subbasins.

In addition, the estimation of potential runoff-contributing areas was limited in this study in that only topographic and soil characteristics were considered. The incorporation of additional factors such as land use and climatic variability may improve the results. For example, an overlay analysis to determine the location of cropland with respect to potential contributing areas may identify the most likely source areas of certain nonpoint-source pollutants within a subbasin. Such information may provide additional guidance for the targeting of BMP's.

REFERENCES

Dunne, Thomas, Moore, T.R., and Taylor, C.H., 1975, Recognition and prediction of runoff-producing zones in humid regions: Hydrological Sciences Bulletin, v. 20, p. 305-327.

Hornberger, G.M., Raffensperger, J.P., Wiberg, P.L., and Eshleman, K.N., 1998, Elements of physical hydrology: Baltimore, MD, The Johns Hopkins University Press, 302 p.

Jenson, S.K., and Domingue, J.D., 1988, Extracting topographic structure from digital elevation data for geographic information system analysis: Photogrammetric Engineering and Remote Sensing, v. 54, p. 1593-1600.

Juracek, K.E., 1999, Estimation of potential runoff-contributing areas in the Kansas--Lower Republican River Basin, Kansas: U.S. Geological Survey Water-Resources Investigations Report 99-4089, 24 p.

Kansas Applied Remote Sensing Program, 1993, Kansas land cover data base, 1:100,000 scale: Kansas Data Access and Support Center data available on the World Wide Web, accessed December 9, 1998, at URL http://ksgis.kgs.ukans.edu/kanview/kanview.html

Moody, D.W., Chase, E.B., and Aronson, D.A., compilers, 1986, National water summary 1985--hydrologic events and surface-water resources: U.S. Geological Survey Water-Supply Paper 2300, 506 p.

Paulson, R.W., Chase, E.B., Roberts, R.S., and Moody, D.W., compilers, 1991, National water summary 1988-89--hydrologic events and floods and droughts: U.S. Geological Survey Water-Supply Paper 2375, 591 p.

Soil Survey Staff, 1997, Natural Resources Conservation Service, National soil survey handbook, title 430-VI: Washington, D.C., U.S. Government Printing Office, December 1997, available on the World Wide Web, accessed November 20, 1998, at URL http://www.statlab.iastate.edu/soils/nssh/

U.S. Department of Agriculture, 1993, State soils geographic data base (STATSGO) users guide: Soil Conservation Service Miscellaneous Publication 1492, 88 p.

U.S. Department of Agriculture, Natural Resources Conservation Service, 1997, 11- and 14-digit hydrologic unit boundaries for Kansas, 1:24,000 scale: Kansas Data Access and Support Center data available on the World Wide Web, accessed August 11, 1998, at URL http://ksgis.kgs.ukans.edu/dasc.html

U.S. Environmental Protection Agency, 1991, Guidance of water quality-based decisions-the TMDL process: Washington, D.C., Office of Water, EPA440/4-91-001, 59 p.

Verdin, K.L., and Greenlee, S.K., 1996, Development of continental scale digital elevation models and extraction of hydrographic features, in Proceedings, Third International Conference/Workshop on Integrating GIS and Environmental Modeling, Sante Fe, New Mexico, January 21-26, 1996: Santa Barbara, CA, National Center for Geographic Information and Analysis, available on compact disk.

Wolock, D.M., and McCabe, G.J., 1995, Comparison of single and multiple flow-direction algorithms for computing topographic parameters in TOPMODEL: Water Resources Research, v. 31, p. 1315-1324.

For additional information on the TMDL project in Kansas, please visit: http://ks.water.usgs.gov/pages/tmdl

For additional information contact:

Kyle Juracek
U.S. Geological Survey
4821 Quail Crest Place
Lawrence, KS 66049
Telephone: (785) 832-3527
Fax: (785) 832-3500
Email: kjuracek@usgs.gov