A conceptual, continuous time model called SWAT (Soil and Water Assessment Tool) was developed to assist water resource managers in assessing the impact of management on water supplies and nonpoint source pollution in watersheds and large river basins. The model is currently being utilized in several large area projects by EPA, NOAA, NRCS and others to estimate the off-site impacts of climate and management on water use, nonpoint source loadings, and pesticide contamination. Model development, operation, limitations, and assumptions are discussed and components of the model are described. In Part II, a GIS input/output interface is presented along with model validation on three basins within the Upper Trinity basin in Texas.

Large Area Hydrologic Modeling and Assessment Part i: Model Development

The topography of a catchment has a major impact on the hydrological, geomorphological. and biological processes active in the landscape. The spatial distribution of topographic attributes can often be used as an indirect measure of the spatial variability of these processes and allows them to be mapped using relatively simple techniques. Many geographic information systems are being developed that store topographic information as the primary data for analysing water resource and biological problems. Furthermore, topography can be used to develop more physically realistic structures for hydrologic and water quality models that directly account for the impact of topography on the hydrology. Digital elevation models are the primary data used in the analysis of catchment topography. We describe elevation data sources, digital elevation model structures, and the analysis of digital elevation data for hydrological, geomorphological, and biological applications. Some hydrologic models that make use of digital representations of topography are also considered.

Digital terrain modelling: A review of hydrological, geomorphological, and biological applications

A framework is provided for scaling and scale issues in hydrology. The first section gives some basic definitions. This is important as researchers do not seem to have agreed on the meaning of concepts such as scale or upscaling. ‘Process scale’, ‘observation scale’ and ‘modelling (working) scale’ require different definitions. The second section discusses heterogeneity and variability in catchments and touches on the implications of randomness and organization for scaling. The third section addresses the linkages across scales from a modelling point of view. It is argued that upscaling typically consists of two steps: distributing and aggregating. Conversely, downscaling involves disaggregation and singling out. Different approaches are discussed for linking state variables, parameters, inputs and conceptualizations across scales. This section also deals with distributed parameter models, which are one way of linking conceptualizations across scales. The fourth section addresses the linkages across scales from a more holistic perspective dealing with dimensional analysis and similarity concepts. The main difference to the modelling point of view is that dimensional analysis and similarity concepts deal with complex processes in a much simpler fashion. Examples of dimensional analysis, similarity analysis and functional normalization in catchment hydrology are given. This section also briefly discusses fractals, which are a popular tool for quantifying variability across scales. The fifth section focuses on one particular aspect of this holistic view, discussing stream network analysis. The paper concludes with identifying key issues and gives some directions for future research.

/pdf/scale-issues-in-hydrological-modelling-a-review-1x61lpwqjr.pdf

Scale issues in hydrological modelling: A review

[1] The science of hydrology is on the threshold of major advances, driven by new hydrologic measurements, new methods for analyzing hydrologic data, and new approaches to modeling hydrologic systems. Here I suggest several promising directions forward, including (1) designing new data networks, field observations, and field experiments, with explicit recognition of the spatial and temporal heterogeneity of hydrologic processes, (2) replacing linear, additive “black box” models with “gray box” approaches that better capture the nonlinear and non-additive character of hydrologic systems, (3) developing physically based governing equations for hydrologic behavior at the catchment or hillslope scale, recognizing that they may look different from the equations that describe the small-scale physics, (4) developing models that are minimally parameterized and therefore stand some chance of failing the tests that they are subjected to, and (5) developing ways to test models more comprehensively and incisively. I argue that scientific progress will mostly be achieved through the collision of theory and data, rather than through increasingly elaborate and parameter-rich models that may succeed as mathematical marionettes, dancing to match the calibration data even if their underlying premises are unrealistic. Thus advancing the science of hydrology will require not only developing theories that get the right answers but also testing whether they get the right answers for the right reasons.

/pdf/getting-the-right-answers-for-the-right-reasons-linking-3sdguqy406.pdf

Getting the right answers for the right reasons: Linking measurements, analyses, and models to advance the science of hydrology

The Prediction in Ungauged Basins (PUB) initiative of the International Association of Hydrological Sciences (IAHS), launched in 2003 and concluded by the PUB Symposium 2012 held in Delft (23–25 October 2012), set out to shift the scientific culture of hydrology towards improved scientific understanding of hydrological processes, as well as associated uncertainties and the development of models with increasing realism and predictive power. This paper reviews the work that has been done under the six science themes of the PUB Decade and outlines the challenges ahead for the hydrological sciences community.Editor D. KoutsoyiannisCitation Hrachowitz, M., Savenije, H.H.G., Bloschl, G., McDonnell, J.J., Sivapalan, M., Pomeroy, J.W., Arheimer, B., Blume, T., Clark, M.P., Ehret, U., Fenicia, F., Freer, J.E., Gelfan, A., Gupta, H.V., Hughes, D.A., Hut, R.W., Montanari, A., Pande, S., Tetzlaff, D., Troch, P.A., Uhlenbrook, S., Wagener, T., Winsemius, H.C., Woods, R.A., Zehe, E., and Cudennec, C., 2013. A d...

/pdf/a-decade-of-predictions-in-ungauged-basins-pub-a-review-2g78unzpne.pdf

A decade of Predictions in Ungauged Basins (PUB)—a review

An automated method of partitioning catchments into interconnected elements using a “stream tube” approach and vector or contour-based digital elevation models is briefly described. With this form of partitioning, hydrologic models can be structured based on the hydraulics of flow within a catchment and the effects of topography on runoff producing mechanisms and spatially distributed flow characteristics (such as flow depth and velocity) can be directly, and realistically, accounted for in the models. The method allows complex three-dimensional flow problems to be reduced to a series of coupled one-dimensional problems in areas with complex terrain. Two simple process-oriented hydrologic models that demonstrate the utility of this form of partitioning are presented. The first models subsurface flow-saturation overland flow and the second models Hortonian overland flow. Observed and predicted runoff hydrographs and the dynamic expansion and contraction of runoff source areas on a small laboratory microcatchment are presented. Also shown are predicted runoff hydrographs and surface flow velocities on a small rangeland catchment in the United States.

Terrain-based catchment partitioning and runoff prediction using vector elevation data

Five mathematical models for predicting subsurface flow were compared to discharge measurements made by Hewlett and Hibbert (1963) on a uniform sloping soil trough at the Coweeta Hydrologic Laboratory. The models included one- and two-dimensional finite element models based on the Richards equation, a kinematic wave model, and two simple storage-discharge models based on the kinematic wave and Boussinesq assumptions. The simple models simulated the subsurface response and water table positions as well as the more complex models based on the Richards equation and were much more economical to use from the point of view of computational costs. Such models have features that would allow them to be incorporated into more complex watershed models, thus placing hydrologic prediction on a more physically correct and less empirical footing.

Modeling subsurface stormflow on steeply sloping forested watersheds

A simple conceptual rainfall-runoff model, based on the variable source area concept, was developed for predicting runoff from small, steep-sloped, forested Appalachianwatersheds. Tests of the model showed that the predicted and observed daily discharges were in good agreement. The results demonstrate the ability of the model to simulate the "flashy" hydrologic behavior of these watersheds. Five subsurface flow models were evaluated by application to existing data measured at Coweeta on a reconstructed homogeneous forest soil. The five models were: Nieber's 2-D and 1-D finite element models (based on Richards' equation), the kinematic wave equation, and two simple storage models developed by the authors, the Boussinesq and kinematic storage models. All five models performed reasonably well on this homogeneous soil. The coupled infiltration model had a large effect on the simulation results. The cost of running the computer models and the computer memory requirements increased as their complexity increased. Field soil-water and precipitation measurements were made on a small test plot in Robinson Forest, in Eastern Kentucky. These data were used to calculate runoff during four precipitation events and to test three of the suteliffac^~flow~"mo3ers~"6h""'a''Tiat:lir^~^atershed. Of the models tested, the simple kinematic storage model performed the best. Flow from the test plot was dominated by macropore flow during storm events, and by flow through the soil matrix during baseflow or recession periods. No surface runoff was observed on the test plot during the period of field observations, except on the saturated near-channel source areas; all runoff was initiated by subsurface flow. Descriptors: Forest watershed*; forest hydrology; mathematical models; model testing; subsurface flow*; subsurface water; runoff. Identifiers: Macropore flow; stormflow; steeply-sloping forested watersheds; watershed models; process models.

https://uknowledge.uky.edu/cgi/viewcontent.cgi?article=1060&context=kwrri_reports

Modeling Surface and Subsurface Stormflow on Steeply-Sloping Forested Watersheds

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