KWRRI Research Reports


The advent of high-speed electronic computer made it possible to model complex hydrologic processes by mathematical expressions and thereby simulate streamflows from climatological data. The most widely used program is the Stanford Watershed Model, a digital parametric model of the land phase of the hydrologic cycle based on moisture accounting processes. It can be used to simulate annual or longer flow sequences at hourly time intervals. Due to its capability of simulating historical streamflows from recorded climatological data, it has a great potential in the planning and design of water resources systems. However, widespread use of the Stanford Watershed Model has been deterred by difficulties in understanding and finding a computer sufficiently large to run the bulky program. More important, the estimation of values for key parameters was both time-consuming and subjective as it had to be done by trial and error.

The objective of this study is to develop a computerized parameter optimization procedure, a self-calibrating watershed model, based on the FORTRAN version of the Stanford Watershed Model known as the Kentucky Watershed Model. This computerized procedure is named OPSET because its objective is to determine an optimum set of parameter values. The basic approach of OPSET is to match synthesized flows with recorded flows. The first step is by sensitivity studies to determine which key watershed para.meters are sensitive in the simulation of flows and are difficult to measure or estimate directly. The second step is to devise a scheme for adjusting numerical estimates of the selected key parameters systematically improving flow simulation until the best possible matching is achieved and to program this scheme into a streamlined Kentucky Watershed Model. Independent adjustment schemes are used for parameters associated with simulating runoff volumes, recession flows and flood hydrograph. The third step is to empirically test and improve this self-calibrating watershed model by applying it to a number of watersheds in Kentucky. OPSET estimates selected watershed parameters on a one water year basis, and the values of parameters best describing the watershed characteristics should be averaged from several OPSET-selected one-year-based values.

In applying OPSET to over 20 Kentucky watersheds which represent quite a wide range of topographic and soil conditions, this model was found to be rather successful. It is able to simulate streamflows and find more consistently estimated para.meter values than the trial-and-error approach. The time spent on calibrating the watershed parameters is greatly reduced. The user does not have to spend so much time familiarizing himself with the program before he can properly use the Model. The program uses standardized criteria which reduce the subjectivity of estimating parameter values.

The recommendation is that OPSET should be applied to areas where the climatological setting and geographical conditions differ from Kentucky in order to refine and modify it for a wider range of applicability. Also, the Model itself needs periodic updating in order to take advantage of subsequent empirical relationships or moisture accounting procedures.

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Funding Information

Part 1 of a completion report describing work supported in part by the Office of Water Resources Research, Department of the Interior, under provisions of Public Law 88-379, as Project Number C-1282 under Title II Research Grant No. 14-01-0001-1964.