Relationship between soil water storage, deep percolation and subsurface return flow (original) (raw)
Related papers
Physically based hydrologic modeling: 2. Is the concept realistic?
Water Resources Research, 1992
Future directions for physically based, distributed-parameter models intended for use as hydrologic components of sediment and nutrient transport models are discussed. The attraction of these models is their potential to provide information about the flow characteristics at points within catchments, but current representations in process-based models are often too crude to enable accurate, a priori application to predictive problems. The difficulties relate to both the perception of model capabilities and the fundamental assumptions and algorithms used in the models. In addition, the scale of measurement for many parameters is often not compatible with their use in hydrologic models. The most appropriate uses of process-based, distributed-parameter models are to assist in the analysis of data, to test hypotheses in conjunction with field studies, to improve our understanding of processes and their interactions and to identify areas of poor understanding in our process descriptions. The misperception that model complexity is positively correlated with confidence in the results is exacerbated by the lack of full and frank discussion of a model's capability/limitations and reticence to publish poor results. This may ultimately diminish the opportunity to advance understanding of natural processes because the managers of research resources are given the impression that the answers are already known and are being provided by models. Model development is often not carried out in conjunction with field programs designed to test complex models, so the link with reality is lost. INTRODUCTION The first paper in this series [Grayson et al., this issue] was concerned with the development, testing and application of a physically based hydrologic model and the investigation, through example, of some barriers to future development of such models. In this paper we consider the conclusions from that work in light of the experience of others and discuss future directions in model use and interpretation as well as the role of field data for model testing. The issues raised were brought into focus via our experience with THALES; however, the conclusions have been generalized through a review of similar studies. While the problems encountered in applications of THALES to the Wagga Wagga and Lucky Hills catchments [Grayson et al., this issue] are model and site specific in one sense, they point to wider issues, and it is these that are dealt with herein. LIMITATIONS OF PHYSICALLY BASED HYDROLOGIC MODELS The assumptions underlying physically based hydrologic models such as THALES are extensive and operate at all levels of the model from the overall model structure to the constituent algorithms. The following discussion considers the conceptual development and process representation of Copyright 1992 by the American Geophysical Union. Paper number 92WR01259. 0043-1397/92/92 WR-01259505.00 physically based, distributed-parameter models intended for use as the hydrological component of sediment and nutrient transport models. Wood et al. [1988, 1990] investigated the existence of a representative elementary area (REA) in the context of hydrologic modeling at the catchmerit scale and defined it as the minimum area within which "implicit continuum assumptions can be used without knowledge of the patterns of parameter values, although some knowledge of the underlying distributions may still' be necessary" [Wood et al., 1988, p. 31]. Topography strongly influences the size of the REA, but for the catchments studied by Wood et al. [1988], the
Hydrological field data from a modeller's perspective: Part 1. Diagnostic tests for model structure
Hydrological Processes, 2011
Hydrological scientists develop perceptual models of the catchments they study, using field measurements and observations to build an understanding of the dominant processes controlling the hydrological response. However, conceptual and numerical models used to simulate catchment behaviour often fail to take advantage of this knowledge. It is common instead to use a pre-defined model structure which can only be fitted to the catchment via parameter calibration. In this article, we suggest an alternative approach where different sources of field data are used to build a synthesis of dominant hydrological processes and hence provide recommendations for representing those processes in a time-stepping simulation model. Using analysis of precipitation, flow and soil moisture data, recommendations are made for a comprehensive set of modelling decisions, including Evapotranspiration (ET) parameterization, vertical drainage threshold and behaviour, depth and water holding capacity of the active soil zone, unsaturated and saturated zone model architecture and deep groundwater flow behaviour. The second article in this two-part series implements those recommendations and tests the capability of different model sub-components to represent the observed hydrological processes.
Physically based hydrologic modeling: 1. A terrain-based model for investigative purposes
Water Resources Research, 1992
THALES, a simple distributed parameter hydrologic model is presented and applied to two catchments in Australia and the United States, each with different dominant hydrologic responses. The model simulates HortonJan overland flow and runoff from saturated source areas and is used to identify some of the barriers to modeling the hydrology of small catchments. At Wagga Wagga in New South Wales, Australia, runoff is produced from saturated source areas, whereas on the Lucky Hills catchmerits at Walnut Gulch in Arizona, Hortonian overland flow processes dominate. Simulations at Wagga Wagga are based on published parameters and field data measured as part of an intensive field program and result in a relatively poor fit of the outflow hydrographs for a series of storms. The simulated position and growth of saturated areas coincides with the limited available information, indicating that at least the gross effects of subsurface water movement are being represented. For the Lucky Hills catchments, the hydrographs at the catchment outlet and points within the catchmerit are simulated for a storm series. The results are highly dependent on the parameter values, which are poorly defined, highlighting the lack of measured field data and lack of methodology for the collection of data at a scale appropriate for such models. The model structure is also shown to have a major influence on the output. The influence of simulating surface flow as sheet flow or rill flow or through a series of ephemeral gullies, as well as the choice of the surface roughness parameter and antecedent soil water conditions, is shown to have a profound effect on the distributed flow depth and velocity predictions. By fitting model parameters, a simulation assuming HortonJan overland flow produced similar results at the catchment outlet to those based on partial area runoff. These results are of concern since it is common to calibrate and verify hydrologic models based on the accuracy with which the catchment outflow is predicted. The internal estimates of flow characteristics following such a calibration often provide the input to sediment and nutrient transport models. Models such as THALES produce an enormous amount of information and have the theoretical potential to provide a "universal" tool for the representation of hydrologic response. However, problems of verification and validation of such models are acute. These problems relate to the difficulty in measuring/deriving parameters a' priori, measurement of the catchment response in sufficient detail for testing, and the validity of the fundamental assumptions and algorithms used in model development. drive the sediment and nutrient transport models that rely on accurate representation of the surface flow. Many aspects of hydrologic response are dependent on topography, and accurate representation of a catchment's topographic characteristics is fundamental to the accurate
Hydrological Processes, 2011
Hydrological scientists develop perceptual models of the catchments they study, using field measurements and observations to build an understanding of the dominant processes controlling the hydrological response. However, conceptual and numerical models used to simulate catchment behaviour often fail to take advantage of this knowledge. It is common instead to use a pre-defined model structure which can only be fitted to the catchment via parameter calibration. In this article, we suggest an alternative approach where different sources of field data are used to build a synthesis of dominant hydrological processes and hence provide recommendations for representing those processes in a time-stepping simulation model. Using analysis of precipitation, flow and soil moisture data, recommendations are made for a comprehensive set of modelling decisions, including Evapotranspiration (ET) parameterization, vertical drainage threshold and behaviour, depth and water holding capacity of the active soil zone, unsaturated and saturated zone model architecture and deep groundwater flow behaviour. The second article in this two-part series implements those recommendations and tests the capability of different model sub-components to represent the observed hydrological processes.
Hydrological models: mathematics or science?
Hydrological Processes, 2010
Within the last 40 years or so there have been a large number of contributions to the scientific literature (journal articles, conference presentations and books) on various aspects of catchment hydrology (rainfall-runoff) models. The focus of these contributions has been on the structure of the models (both mathematical and hydrological), the software used to implement them, methods of calibration (both manual and automatic), estimation of parameters in ungauged situations, their practical application for solving problems and, with increasing frequency in recent years, the uncertainties associated with their outputs. These models are generally defined as mathematical representations of the hydrological cycle at the catchment scale and it is interesting to note how many of the contributions focus on the 'mathematical' part of that definition and how many on the 'hydrological' part. More than 20 years ago, Klemeš (1986) contested that for '.. . a good hydrological model it is not enough to work well. It must work well for the right reasons'. The main focus of this commentary is whether or not that message is true, whether active researchers and practitioners recognize it as an issue and whether or not it is possible to decide if a model is working for the right reasons in ungauged (i.e. no observed streamflow data) catchments. Does the level of complexity influence whether models can work well for the right reasons? The argument for so-called 'physics-based' models arose partly from the desire to include hydrological processes more explicitly (Abbott et al., 1986). However, there seems to be a point at which added complexity in the model structure is not matched by our ability to quantify the model parameters realistically, given typically available data resources. This is an important consideration in data scarce areas which are often the very areas where hydrological models have the potential to provide valuable information for the purposes of water resources planning and management. Beven (1989) questioned the use of algorithms based on small-scale process observations in models that are used at much larger scales. At the other end of the complexity scale, it is perhaps difficult to imagine how it is possible to assess whether highly simplified models are doing the right thing for the right reason. Simple models may be easier to calibrate, particularly when automatic calibration approaches (Vrugt et al., 2003) are being used, but this then becomes a largely mathematical fitting exercise. The inevitable process lumping that occurs in simple models implies that the parameters have little physical meaning, are just mathematical constants and are difficult to extrapolate to ungauged catchments. The desire for parsimonious models (Jakeman and Hornberger, 1993; Perrin et al., 2001), expressed by some contributions to the literature, must surely emanate from the school of hydrological modellers who are looking for elegant mathematical solutions to a complex input (i.e. climate signals)-output (streamflow) problem. It has been claimed in the past (Loague and Freeze, 1985) that more complex models have too many parameter interactions and are fraught with problems of equifinality and lack of parameter identifiability (Beven, 2006), whereby different parameter sets can result in very similar streamflow
Development and application of a simple hydrogeomorphic model for headwater catchments
Water Resources Research, 2011
We developed a catchment model based on a hydrogeomorphic concept that simulates discharge from channel-riparian complexes, zero-order basins (ZOB, basins ZB and FA), and hillslopes. Multitank models simulate ZOB and hillslope hydrological response, while kinematic wave models predict saturation overland runoff from riparian zones and route inputs from ZOB and riparian corridors through the channel. The model was parameterized and tested in the Hitachi Ohta Experiment Watershed, Japan. Tank models were parameterized for a 6 month period from May to October 1992, and these models were then tested for the same 6 month period in 1993. In ZB, with relatively shallower soils, total outflow for the 6 month period in 1993 was underpredicted by 25%. Better predictions were obtained for outflow from FA (deeper soils; À17%) and the entire catchment (À5%). Total runoff from the channel and riparian area depends on the ratio of this area to the total catchment area because this corridor is assumed to be saturated at all times. Stormflow response from ZOB was limited during relatively dry conditions and increased substantially during wetter conditions, especially in ZB, which has shallower soils (1.4 m of average); such effects were diminished in FA (deeper soils) and hillslopes. Outflow from ZB had the highest proportion of rapid flow, while slower flow dominates outflow from FA and hillslopes; these different responses appear to be mainly associated with soil depth and topography. Groundwater recharge, estimated by leakage from the lowermost tank in the models, was as high as 61 mm week À1 from ZB, with lesser recharge from other geomorphic components (18-21 mm week À1). These spatially explicit simulations provide a simpler approach to the greater data demands of distributed hydrologic models without compromising process function.
Infl uence of Soil Heterogeneity and Spa al Discre za on on Catchment Water Balance Modeling
This study inves gated the impacts of the spa al variability of soil hydraulic proper es and the eff ects of spa al discre za on on the water balance in a fully coupled system. The integrated surface-subsurface, three-dimensional, fi nite element model HydroGeoSphere was applied to the forested Wüstebach basin (27 ha) to simulate water fl uxes. The fully coupled fl ow simula on model was applied to the headwater catchment at two diff erent spa al resolu ons (25 and 100 m). The change in spa al resolu on required an aggrega on of the soil map, which infl uenced the water fl uxes and the spa al pa erns of soil moisture. The nonlinear rela onship between soil moisture and transpira on caused the spa al aggrega on of soil moisture to have a larger eff ect on the water balance than did aggrega ng the soil hydraulic proper es. In addi on to the total discharge, the eff ects on the spa al pa erns of the simulated soil moisture were also inves gated. The results show that aggrega ng soil hydraulic proper es results in lower uncertain es than does using a coarser discre za on. This can be explained by the nonlinearity of the rela onship between soil moisture and evapotranspira on.
A framework for development and application of hydrological models
2001
Abstract Many existing hydrological modelling procedures do not make best use of available information, resulting in non-minimal uncertainties in model structure and parameters, and a lack of detailed information regarding model behaviour. A framework is required that balances the level of model complexity supported by the available data with the level of performance suitable for the desired application.
A detailed model for simulation of catchment scale subsurface hydrologic processes
Water Resources Research, 1993
A catchment scale numerical model is developed based on the three-dimensional transient Richards equation describing fluid flow in variably saturated porous media. The model is designed to take advantage of digital elevation data bases and of information extracted from these data bases by topographic analysis. The practical application of the model is demonstrated in simulations of a small subcatchment of the Konza Prairie reserve near Manhattan, Kansas. In a preliminary investigation of computational issues related to model resolution, we obtain satisfactory numerical results using large aspect ratios, suggesting that horizontal grid dimensions may not be unreasonably constrained by the typically much smaller vertical length scale of a catchment and by vertical discretization requirements. Additional tests are needed to examine the effects of numerical constraints and parameter heterogeneity in determining acceptable grid aspect ratios. In other simulations we attempt to match the observed streamflow response of the catchment, and we point out the small contribution of the streamflow component to the overall water balance of the catchment. the Konza Prairie Research Natural Area near Manhattan, Kansas. The practical application of our model to actual catchments is demonstrated, and we investigate computa-•Now at CRS4, on large-scale simulation of hydrologic processes. One of the overriding problems in hydrology is the understanding of responses over the range of scales O(10 -•) to O(104) km 2 [Wood et al., 1988; Goodrich and Woolhiser, 1991]. The low end of this range is roughly the size of a small subcatchment and marks a transition from point and hillslope scales, at which physically based hydrologic models are more easily tested and better understood, to catchment or basin scales. The high end corresponds to the horizontal grid scale used in general circulation models for global climate simulations, and a better understanding of hydrologic processes at this scale is required to improve the land surface boundary conditions for these models. The parameterization of hydrologic processes at large scales is made difficult by the high degree of nonlinearity and variability in catchment parameters and inputs, and thus conceptual or idealized models are often used at these scales. Physically based analytical or numerical models can be used to study the validity of simplifying assumptions in conceptual models. Sorne examples can be found in the works by Reeves and Miller [1975] (time compression approximation for partitioning rainfall into runoff and infiltration), Broadbridge and White [1987] (time to ponding), Gan and Burges [1990a, b] (catchment rainfall-runoff models), Shamsai and Narasimhan [1991] (Dupuit-Forchheimer assumption under seepage face conditions), Sloan and Moore [1984] and Stagnittiet al. [1986] (subsurface flow models), Wilcox et al. [1990] (runoff prediction models), and Troch et al. [1993] (catchment scale water balance models). Other studies using physically based hillslope and catchment scale models include the early work of Freeze [1971, 1972a, b], who used a three-dimensional finite difference variably saturated flow model coupled with a onedimensional channel flow model to reveal the importance of subsurface flow processes and parameter variability on watershed runoff response. Smith and Hebbert [1983] sim-
IHMS-Integrated Hydrological Modelling System. Part 1. Hydrological processes and general structure
Hydrological Processes, 2010
A newly Integrated Hydrological Modelling System (IHMS) has been developed to study the impact of changes in climate, land use and water management on groundwater and seawater intrusion (SWI) into coastal areas. The system represents the combination of three models, which can, if required, be run separately. It has been designed to assess the combined impact of climate, land use and groundwater abstraction changes on river, drainage and groundwater flows, groundwater levels and, where appropriate, SWI. The approach is interdisciplinary and reflects an integrated water management approach. The system comprises three packages: the Distributed Catchment Scale Model (DiCaSM), MODFLOW (96 and 2000) and SWI models. In addition to estimating all water balance components, DiCaSM, produces the recharge data that are used as input to the groundwater flow model of the US Geological Survey, MODFLOW. The latter subsequently generates the head distribution and groundwater flows that are used as input to the SWI model, SWI. Thus, any changes in land use, rainfall, water management, abstraction, etc. at the surface are first handled by DiCaSM, then by MODFLOW and finally by the SWI. The three models operate at different spatial and temporal scales and a facility (interface utilities between models) to aggregate/disaggregate input/output data to meet a desired spatial and temporal scale was developed allowing smooth and easy communication between the three models. As MODFLOW and SWI are published and in the public domain, this article focuses on DiCaSM, the newly developed unsaturated zone DiCaSM and equally important the interfacing utilities between the three models. DiCaSM simulates a number of hydrological processes: rainfall interception, evapotranspiration, surface runoff, infiltration, soil water movement in the root zone, plant water uptake, crop growth, stream flow and groundwater recharge. Input requirements include distributed data sets of rainfall, land use, soil types and digital terrain; climate data input can be either distributed or non-distributed. The model produces distributed and time series output of all water balance components including potential evapotranspiration, actual evapotranspiration, rainfall interception, infiltration, plant water uptake, transpiration, soil water content, soil moisture (SM) deficit, groundwater recharge rate, stream flow and surface runoff. This article focuses on details of the hydrological processes and the various equations used in DiCaSM, as well as the nature of the interface to the MODFLOW and SWI models. Furthermore, the results of preliminary tests of DiCaSM are reported; these include tests related to the ability of the model to predict the SM content of surface and subsurface soil layers, as well as groundwater levels. The latter demonstrates how the groundwater recharge calculated from DiCaSM can be used as input into the groundwater model MODFLOW using aggregation and disaggregation algorithms (built into the interface utility). SWI has also been run successfully with hypothetical examples and was able to reproduce the results of some of the original examples of . In the subsequent articles, the results of applications to different catchments will be reported.