1. Line segments are the graphical entities necessary to represent basin boundaries and watercourses. Linear features can be made from several segments, but segments must have endpoints where segments contact each other. Line segments can be joined together to represent a linear feature (watercourse) or form a closed area called a polygon (subbasin).
2. Subbasins are represented by polygons that are uniquely identified by the GIS.
3. Subbasin boundaries can be delineated beginning at any point on a stream. The boundaries typically begin at the confluence of major tributaries, streamflow gaging stations, sewage-treatment-plant outfalls, and mouths of major tributaries to lakes.
4. Subbasins that do not contribute directly to overland runoff be delineated and identified by the user.
5. The watercourse data structure is a dichotomous, non-braided, nondivergent, connected, dendritic representation of the stream system.
6. An individual watercourse is defined as a single line from its confluence or mouth to the basin divide. At each branch of the stream, the line follows the branch draining the largest area. The line is continuous and passes through marshes, lakes, and the midline of wide rivers and braided streams. The watercourse has a unique, arbitrary identifier assigned by the integration program.
7. The segments of the watercourse that lie within a subbasin also have an identifier assigned by the GIS that associates the segment with the subbasin.

Methods

A hypothetical drainage basin containing streams, lakes, and marsh areas is shown in figure 1. An example of how the basin in figure 1 may be divided by the user into subbasins is shown in figure 2. The example shows the segments representing the watercou rses and their respective segment numbers and the subbasin boundaries and their respective basin numbers. Lists of the data generated by the integration program for the example in figure 2 are contained in tables 1 and 2.

The downstream-order number in table 1 is an arbitrary number assigned to a subbasin. The number increases from the uppermost subbasin on the main stream down to the basin outlet. Downstream-order numbers are dependent on the number of watercourse segment s and noncontributing basins and, therefore, not necessarily sequential. The number also increases from the uppermost subbasin of a tributary down to the tributary outlet. This way of assigning numbers to basins provides a technique for easily aggregating data for basins because only those subbasins that contribute flow have a downstream-order number between numbers for the uppermost subbasin and the outlet subbasin.

The stream identifier in table 2 is an arbitrary number assigned to the watercourse that drains the larger area. This number defines which segments are selected to calculate stream characteristics such as main-channel length.

The cumulative area of a segment is defined as the total area of the drainage subbasins upstream of the downstream end of the segment. If the subbasin has many segments, the area of the subbasin is distributed proportionally according to the lengths of th e segments within that subbasin.

The modified-Horton-order number is a description of the morphology of a stream system. A modified-Horton-order number of 1 designates the main stream of the system. Tributaries to the main stream have modified-Horton-order numbers of 2, and so forth.

The Strahler number is another description of the morphology of the stream system. A Strahler number of 1 indicates that the stream is the uppermost tributary. The Strahler number increases by 1 at the confluence of two streams with equal Strahler numbers . The Strahler number is the greater of the two where two streams of unequal Strahler numbers join.

Special Conditions

A subbasin that does not normally contribute runoff to the stream system, called a noncontributing subbasin, can be incorporated into the stream-drainage system by assigning the subbasin to a segment of a watercourse-or a noncontributing subbasin can be o mitted. The integration program checks each segment or noncontributing subbasins that have been assigned to it. If a segment has a noncontributing subbasin assigned to it, the area of that subbasin is added to the cumulative area for that segment. Noncon tributing subbasins can be omitted by not assigning the area to a segment; the cumulative area then represents only subbasins contributing flow to the stream system. Subbasin number 8 was assigned to stream-segment number 6 in figure 3.

Subbasins delineated upstream of the junction of the watercourses, called isolated subbasins, occur commonly for large tributaries to lakes; for example, subbasin 9 in figure 3. Segment number 11 does not properly belong to subbasin 6, and the cumulative area for segment 11 must represent only the drainage area upstream of the outlet of subbasin 9. The integration program checks each segment to determine if the area of the subbasin should not be accumulated. The stream system must be checked by the user for segments that drain isolated subbasins.

Lists of data for the example of the noncontributing subbasin and the isolated subbasin shown in figure 3 are contained in tables 3 and 4.

The method of distributing the subbasin area according to segment length can produce errors where a large part of the subbasin drains into a short segment. Segment number 10 in figure 3 is much shorter than segment number 8 and should be allocated a larger portion of the subbasin area. Basins with many tributaries must be checked by the user to determine if the distribution of subbasin area has affected the stream-numbering system.

STREAM-DRAINAGE AGGREGATION

The Strahler and modified-Horton-order numbers are by-products of the integration program and are not used by the aggregation program. These numbers do characterize the morphology of the stream, however, and could be used in a hydrological analysis that incorporates stream morphology. The aggregation program requires the downstream-order number and the uppermost downstream-order numbers, selects those subbasins in that range (for each subbasin), and calculates the drainage characteristics for each subbasi n.

The aggregation program requires that the polygons representing lakes and marsh areas be associated with a subbasin. The GIS performs this association and totals the areas within each subbasin. The program steps through the data one subbasin at a time, ca lculates the percentage of area covered by lake and marsh areas within the basin, selects all the subbasins contributing flow to that subbasin, and calculates the cumulative drainage area and the percentage of area covered by lake and marsh areas. A list of drainage area, percentage lake area, and percentage storage area for the subbasins in figure 3 is contained in table 5.

SUMMARY

REFERENCE CITED