Journal of Hydrologic Engineering

Mathematical modeling of watershed hydrology is employed to address a wide spectrum of environmental and water resources problems. A historical perspective of hydrologic modeling is provided, and new developments and challenges in watershed models are discussed. These include data acquisition by remote sensing and space technology, digital terrain and elevation models, chemical tracers, geographic information and data management systems, topographic representation, upscaling of hydrologic conservation equations, spatial variability of hydraulic roughness, infiltration and precipitation, spatial and temporal scaling, model calibration, and linking with water quality models. Model construction, calibration, and data processing have received a great deal of attention, while model validation, error propagation, and analyses of uncertainty, risk, and reliability have not been treated as thoroughly. Finally, some remarks are made regarding the future outlook for watershed hydrology modeling.

Data collected at two measurement scales from a semiarid grassland are presented and analyzed to explore the hydrologic effects of grazing, interactions between overland flow and infiltration, and scale issues. Rainfall-runoff simulations were used to quantify the areal (3 by 10 m plot scale) response, and small-diameter (9 cm) disk infiltrometers were used to estimate point-scale hydraulic conductivity of bare and vegetated soil. Plot-scale data show that grazing increases runoff overall, a common result, but infiltrometer measurements indicate that only the point-scale hydraulic conductivity of vegetated soil is changed by grazing. In light of this and the well-known relationship between microtopography and vegetation in semiarid grasslands, we hypothesize that small-scale surface interactions (in particular, the so-called run-on phenomenon) are a significant component of the observed effects of grazing, as well as a factor in the hydrologic response of grasslands. Results obtained from high-resolution numerical simulations support this hypothesis. This phenomenon is not captured by classical infiltration theory or by the usual methods of statistical parameterization. In general, interactions are more likely to be important as spatial variability increases, and the relative importance of these interactions will be a function of the spatial structure of the variability and the hydrodynamics of overland flow, as controlled by microtopography.

A distributed watershed model—is developed for coupling simulation of hydrological and energy processes in watersheds with complex land covers. In the model, the state variables include depression storage on land surfaces or canopies, soil moisture content, land surface temperature, groundwater level, and river water stage. The subgrid heterogeneity of land use is also taken into consideration using a mosaic method. Estimation methods are proposed for impervious ratio of land use and parameters of soil, aquifer, and vegetation. The model is applied to the Ebi watershed in Japan and validated through comparing simulated river discharges, groundwater levels, and surface temperatures with observed values. In addition, impact of urban development on water budgets is shown through comparing two simulation results for present and future land uses. Lastly, mitigation alternatives of installing infiltration trenches and/or storm-water detention ponds are examined. Joint implementation of infiltration trenches and storm-water detention ponds is suggested for improving the hydrological cycle in the watershed.

The operation of reservoirs in the Sierra Nevada mountains of California for flood control relies on forecasts of reservoir inflows. In the past, accurate forecasts of the reservoir inflows resulting from watershed runoff have been made, but only after the water has entered the main channel. During flooding events, this limits the amount of time available for the implementation of emergency management procedures. Translating precipitation forecasts into runoff forecasts can greatly improve the runoff-forecast lead time. The operational National Center for Environmental Prediction Eta model provides 48-h-ahead forecasts of precipitation in 6-h intervals in a $40\times 40\mathrm{km}$ gridded form. In this study, the mesoscale model, MM5, is used to transfer the Eta forecast data down to the appropriate space and time scales required to link the Eta model precipitation forecast results to the watershed model, HEC-HMS, for runoff prediction. An initial diagnostic study of this procedure has been performed on the Calaveras River watershed in Northern California. Initial results indicate that: (1) model parameterization choice in MM5 is necessary to refine the precipitation forecasts; (2) the method shows promise for generating 48-h-ahead forecasts of reservoir inflows; and (3) calibration of the HEC-HMS model with distributed precipitation is necessary for this methodology. This paper presents the study results along with a discussion of the methodology.

Concerns about protecting first-order receiving streams, improving water quality, and on-site control of storm runoff create a need to extend commonly used hydrologic design methods for application to microwatersheds. Existing design methods cannot adequately be extrapolated for use on small lot-sized watersheds. This research extends the widely used Natural Resources Conservation Service TR-55 design procedures for use on microwatersheds. Specifically, the graphical peak discharge estimation method is extended so that it can be used for times of concentration as small as 0.02 h. The kinematic-wave time of concentration estimation method is made applicable for multiple-section sheet flow, and a new pond-and-swamp adjustment procedure enables the design of small on-site bioretention ponds. The latter design method uses the actual storage depth and can be applied for both partially and fully contained storage.

A fuzzy rule-based methodology is developed for estimating monthly groundwater recharge using the Toms River basin in New Jersey as a case study. As an alternative to a water budget approach, which depends upon difficult-to-quantify parameters, the fuzzy methodology simplifies model input by using air temperature as a surrogate for evapotranspiration and streamflow as a surrogate for both soil-moisture deficit and direct runoff. The accuracy of the fuzzy rule-based method is compared with that of ordinary linear regression. It is found that for the most accurate fuzzy rule-based model, the mean percent difference between its estimate and the “actual” recharge is 14.0%, slightly larger than the 12.5 mean % difference achieved with linear regression. However, as noise was synthetically introduced into the input data, the difference in predictive accuracy between fuzzy rule-based modeling and linear regression decreased further. In addition, unlike linear regression, neural networks, or physically based models, the transparent nature of the fuzzy rules provides explicit qualitative and quantitative insights into historic system behavior and may be used to forecast recharge under variable hydrologic and climatic conditions.

The transport of reactive chemicals through porous media is generally affected by multiple reactions with different rate constants. In this work, we investigate one-dimensional transport affected by rate-limited sorption, first-order mass transfer, and first-order transformation and derive analytical solutions for the spatial moments of the solute in the solution phase using an exponentially increasing dispersivity function to account for the spatial heterogeneity of hydraulic conductivity. Illustrative examples are used to discuss the effect of various rate constants on the travel distance, variance, and skewness. It is shown that the spatial heterogeneity of material properties can be satisfactorily accounted for by using an increasing macrodispersivity function. These solutions will be useful as a preliminary analysis tool for ascertaining the relative importance of various processes under given conditions. They may also be used to analyze the accuracy of various numerical techniques used for simulation of reactive transport through homogeneous and heterogeneous porous media.