Journal of Hydrologic Engineering

The need to simulate surface water flows in watersheds with diverse runoff production mechanisms has prompted the Department of Defense to invest in the development of a new, physically based hydrologic model, called the Gridded Surface/Subsurface Hydrologic Analysis (GSSHA) model. GSSHA is a reformulation and enhancement of the two-dimensional, physically based model CASC2D. The GSSHA model simulates stream flow generated by both infiltration-excess and saturation-excess mechanisms, as well as exfiltration, and groundwater discharge to streams. The model employs mass-conserving solutions of partial differential equations and closely links the hydrologic components to assure an overall mass balance. Testing of the model on a watershed with diverse runoff mechanisms indicates that the model is useful for investigating the important streamflow producing processes at the watershed scale and determining the contributions of surface water, saturated groundwater, and unsaturated groundwater to the overall water balance. Split-sample tests indicate that the model is capable of reproducing both observed hourly and total discharge at the test basin outlet.

The development of model parametrization methodologies using geographic information systems is becoming increasingly important in hydrologic modeling applications, especially given the continued trend of comprehensive and readily available geospatial databases. This paper discusses the development of parametrization and calibration methodologies for the 12-parameter soil moisture accounting (SMA) algorithm recently added to the Hydrologic Modeling System (HMS) program by the Hydrologic Engineering Center (HEC). The U.S. Army Corps of Engineers, Nashville District, is considering HMS as a tool for continuous hydrologic simulation in the Cumberland River basin. The study demonstrates that parameters for a complex soil moisture accounting model can be derived from publicly available geographic information system (GIS) databases and that the use of seasonal or multiparameter sets improves model performance. Only one of the seven parameters estimated using GIS databases was varied during model calibration. Parameter values after model calibration, including four parameters estimated using historic streamflow records, deviated from estimated values by an average of 37%. Model performance indicates that the developed parametrization and calibration methodologies work well when applied to the test watershed and should be applicable to other watersheds in the Cumberland River basin.

Identification of sediment source areas within a watershed is useful for designing appropriate management alternatives that reduce sediment losses. In most watersheds, data collection programs are present at the stream gauging station, and sediment source identification must be treated as an inverse problem. A physically based watershed-scale surface flow and erosion model, KINEROS was utilized as the modeling tool in this study. A modified unit sedimentograph approach was used for source identification based on hydrograph and sedimentograph data collected at the watershed outlet. Data from several events were examined over two small watersheds, and a statistical procedure was employed to assess the erosion vulnerability of different regions in the two watersheds. These results were presented in the form of maps to obtain a spatial picture of potential sediment generating areas. A statistical analysis was also utilized to determine if erosion potentials of different regions of a watershed were significantly different. It appears that identification of source areas is possible when a limited number of regions are contributing sediments.

In this paper, we re-examine the question of identification of source areas using optimization methods for two Iowa watersheds. The question of nonuniqueness is addressed first, with the result that only eight sediment contributing regions are allowed to be active simultaneously as in the companion study. Tukey’s procedure is utilized to examine if the differences in erodibility of the different regions are statistically significant. Results were presented in the form of maps to obtain a spatial picture of the potential of sediment generating areas. Comparisons with physical characteristics of the watersheds showed good agreement between erosion potential and properties associated with erosion.

The purpose of this study is to develop a semidistributed, physically based hydrologic model (SDPB_HM) for mountainous watersheds areas. Most of the model’s required parameters were acquired using remote sensing and digital terrain elevation data. A three-stage computer classifier model was built based on the error back-propagation artificial neural network approach (EBPANN) and refined to classify the land cover types of the study area. In addition, the reflection properties of the land cover types were used to improve the technique of estimating the net radiation in Morton’s evapotranspiration model. A procedure was proposed for discretizing the watershed areas, aimed to increase the homogeneity and minimize the calculation time. The SDPB_HM was applied to the Albert River Basin in the Rocky Mountains in British Columbia, Canada. The Albert River meteorological data from October 1986 to September 1987 was used to calibrate the model parameters. In addition, the SDPB_HM was validated using the meteorological data of a case study from October 1987 to September 1988 and from October 1988 to September 1989. Comparison between the simulated and the observed flow at the outlet of river showed a good agreement during these periods.

Traditional stormwater management does not mitigate groundwater depletion resulting from groundwater pumping and reduction in recharge. Infiltration practices, such as rain gardens, offer a potentially effective approach for addressing groundwater depletion. A rain garden is a landscaped garden in a shallow depression that receives the stormwater from nearby impervious surfaces, focusing recharge. We have developed a numerical model that can be applied in rain garden design and evaluation. Water flow through the rain garden soil is modeled over three layers: a root zone, a middle storage layer of high conductivity, and a subsoil lower layer. To continuously simulate recharge, runoff, and evapotranspiration, the model couples the Richards Equation with a surface water balance. The model was applied to the climate of southern Wisconsin. Simulation results show that very high recharge rates are possible during the non-snowfall season. (The model does not handle snowmelt.) A rain garden with an area of about 10–20% of the contributing impervious area maximizes groundwater recharge. Increasing the depression depth increases recharge and saturation times, affecting plant survival. Rain garden feasibility also depends on the subsoil hydraulic conductivity.

Ice-on and ice-off dates, ice cover duration, and ice thicknesses for four lakes in the United States and one lake in Canada were simulated. The model used is a one-dimensional, deterministic water quality model that simulates daily vertical temperature profiles in lakes year round and has the ability to simulate ice-on dates, ice growth, ice decay, and ice-off dates. The average standard errors between simulated and measured values were 10, 9, and 11 days for ice-on dates, ice-off dates, and ice cover duration, respectively, without calibrating the model. The model was also employed to simulate the effect of a projected $2\times {\mathrm{CO}}_2$ climate scenario on the ice characteristics of the same five lakes. Under the $2\times {\mathrm{CO}}_2$ climate scenario, the average ice-on dates are 7–12 days later; the average ice-off dates are 15–23 days earlier; the ice cover durations are 25–33 days shorter; and the maximum ice thicknesses are 0.18–0.30 m thinner. The sensitivity of lake ice characteristics to different meteorological parameters was also investigated.

Accurate estimates of evapotranspiration from constructed wetlands are required to establish design flow estimates and to assess the effectiveness of wetland water quality amelioration. Water consumption by two wetland plant species, Typha latifoilia (broadleaf cattail) and Scoenoplectus acutus (hardstem bulrush), was measured in a greenhouse for eight months. Measurements of actual evapotranspiration $\left({\mathrm{ET}}_C\right)$ from replicates of both plant treatments were related to potential evaporation $\left({\mathrm{ET}}_0\right)$ as approximated by evaporation from saturated gravel beds. Ratios of ${\mathrm{ET}}_C$ to ${\mathrm{ET}}_0$ were used to develop crop coefficients $\left(K_c\right)$ for each plant species. The relationship between cattail ${\mathrm{ET}}_C/{\mathrm{ET}}_0$ and the ratio of vegetative to open water surface area $(S_V/S_0)$ agreed with previous investigations. A linear relationship was used to account for advective energy fluxes due to peripheral canopy area. Cattail crop coefficients were scaled according to this relationship. The resulting scaled crop coefficient curve may be transferable to constructed wetlands of a known $S_V/S_0\text{.}$

A simple method is suggested for computing the direct runoff hydrograph resulting from a complex storm using a two-parameter gamma distribution as the instantaneous unit hydrograph. The computation of the incomplete gamma function is accomplished indirectly, which requires evaluations of only simple functions that can be performed on a scientific calculator. In place of “unit hydrograph,” a “unit kernel” approach is suggested, which reduces the computational effort when dealing with any selected sampling interval because it implicitly accounts for the sampling interval. The use of the unit kernel renders the S-curve invariant with the sampling interval, as opposed to the conventional procedure. The new procedure enhances the applicability of the two-parameter gamma distribution or the Nash model for rainfall-runoff modeling, as the calculations can be performed using a spreadsheet, such as an Excel spreadsheet. Application of the new procedure on published data sets shows that the direct runoff obtained is almost the same as computed using either tabulated or computer-obtained values of the incomplete gamma function.