Final Report of the DAUFIN project

DAUFIN = Data Assimulation within Unifying Framework for Improved river basiN modeling (EC 5th framework Project

A state-dependent parameterization of saturated-unsaturated zone interaction

The relevance of groundwater as an important source of root zone moisture by means of capillary rise is increasingly being recognized. This is partly reflected in many current land surface schemes, which increasingly replace a one-way (i.e., downward) drainage of water by a two-way interaction flux between the root zone and a groundwater system. A fully physically correct implementation of this two-way saturated-unsaturated interaction flux requires transient simulations using the highly nonlinear Richards' equation, which is a computationally demanding approach. We test a classic simple approximation that computes the root zone¿groundwater interaction flux as the net effect of a downward drainage flux and an upward capillary rise flux against the Darcy equation for quasi steady state conditions. We find that for a wet root zone and/or shallow groundwater, the errors within this approximation are significant and of the same magnitude as the interaction flux itself. We present a new closed-form parameterization of the Darcy equation¿based fluxes that accounts both for root zone soil moisture and depth to the water table. Parameter values for this parameterization are listed for 11 different, widely applied soil texture descriptions. The high numerical efficiency of the proposed method makes it suitable for inclusion into demanding applications, e.g., a Monte Carlo framework, or high spatial resolution

Improved understanding of soil moisture variability dynamics

Different trends of soil moisture variability with mean moisture content have been reported from field observations. Here we explain these trends for three different data sets by showing how vegetation, soil and topography controls interact to either create or destroy spatial variance. Improved understanding of these processes is needed for the transformation of point-scale measurements and parameterizations to scales required for climate studies, operational weather forecasting, and large scale hydrological modeling

Curvature distribution within hillslopes and catchments and its effect on the hydrological response

Topographic convergence and divergence are first order controls on the hillslope and catchment hydrological response, as evidenced by similarity parameter analyses. Hydrological models often do not take convergence as measured by contour curvature directly into account; instead they use comparable measures like the topographic index, or the hillslope width function. This paper focuses on the question how hillslope width functions and contour curvature are related within the Plynlimon catchments, Wales. It is shown that the total width function of all hillslopes combined suggest that the catchments are divergent in overall shape, which is in contrast to the perception that catchments should be overall convergent. This so-called convergence paradox is explained by the effect of skewed curvature distributions and extreme curvatures near the channel network. The hillslope-storage Bossiness (hsB) model is used to asses the effect of within-hillslope convergence variability on the hydrological response. It is concluded that this effect is small, even when the soil saturation threshold is exceeded. Also described in this paper is a novel algorithm to compute flow path lengths on hillslopes towards the drainage network, using the multi-directional flow redistribution metho

On bimodality in warm season soil moisture observations

It has recently been suggested that the bimodality in warm season soil moisture observations in Illinois is evidence of a soil moisture-precipitation feedback. Other studies however provide little evidence for a strong feedback in this region. Here we show that seasonality in the meteorological conditions in combination with the non-linearity of the soil moisture response alone can induce this bimodality. The existence of preferred wet and dry soil moisture states may have implications for the understanding and modeling of soil moisture dynamics in mid-latitude region

Similarity analysis of subsurface flow response of hillslopes with complex geometry

The matter of the efficient and parsimonious parameterization of hillslope subsurface flow remains an important issue in catchment hydrological studies (Brutsaert, 1995). Insights into the influence of the shape and hydraulic characteristics of hillslopes is required to further our understanding and our ability to model catchment hydrological processes. Recently, Troch et al. (2003) introduced the hillslope-storage Boussinesq (HSB) equation to describe subsurface flow and saturation along geometrically complex hillslopes. The HSB equation can be linearized and further reduced to an advection-diffusion equation for subsurface flow in hillslopes with constant bedrock slopes and exponential width functions. This paper presents a dimensional analysis of the latter equation in order to study the moments of the characteristic response function (CRF), corresponding to the free drainage of this type of hillslope. These moments, in a dimensionless form, can be expressed as functions of a similarity parameter, hereafter called the hillslope Péclet number, and a group of dimensionless numbers accounting for the effects of the boundary and initial conditions. The analytical expressions for the first four central CRF moments are derived for two types of initial conditions. The analysis of their respective influences shows that the hillslope Péclet number is an efficient similarity parameter to describe the hillslope subsurface flow response. Moreover, comparison between the CRF moments predicted by means of our similarity analysis and empirical moments derived from outflow measurements for different types of laboratory hillslopes shows good agreemen

Effecten van landgebruiksveranderingen op gemiddelde en extreme afvoer in het Rijnstroomgebied

Recentelijk heeft veel onderzoek plaatsgevonden om de invloed van klimaatverandering te kwantificeren. Dit kan op verschillende manieren gebeuren, bijvoorbeeld kan er op basis van gemeten data een extreme-waardenverdeling worden geëxtrapoleerd. Een dergelijke aanpak heeft als nadeel dat de aanpak gebaseerd is op statistische kenmerken van het huidige klimaat, terwijl die juist waarschijnlijk veranderen. Een andere mogelijkheid is daarom het doorberekenen van klimaatscenario's zoals die worden gegenereerd met klimaatmodellen. Het landoppervlaktemodel dat in deze studie is gebruikt, namelijk het Variable Infiltration Capacity (VIC) model maakt gebruik van statistische parameters voor de invloed van verzadigde bodems. Het gebied betreft de substroomgebieden van Ruhr, Lahn, Mosel, Main en Necka

Analytical solution of the linearized hillslope-storage Boussinesq equation for exponential hillslope width functions

This technical note presents an analytical solution to the linearized hillslope-storage Boussinesq equation for subsurface flow along complex hillslopes with exponential width functions and discusses the application of analytical solutions to storage-based subsurface flow equations in catchment studies

Mixtures of Gaussians for uncertainty description in bivariate latent heat flux proxies

This paper proposes a new probabilistic approach for describing uncertainty in the ensembles of latent heat flux proxies. The proxies are obtained from hourly Bowen ratio and satellite-derived measurements, respectively, at several locations in the southern Great Plains region in the United States. The novelty of the presented approach is that the proxies are not considered separately, but as bivariate samples from an underlying probability density function. To describe the latter, the use of Gaussian mixture density models¿a class of nonparametric, data-adaptive probability density functions¿is proposed. In this way any subjective assumptions (e.g., Gaussianity) on the form of bivariate latent heat flux ensembles are avoided. This makes the estimated mixtures potentially useful in nonlinear interpolation and nonlinear probabilistic data assimilation of noisy latent heat flux measurements. The results in this study show that both of these applications are feasible through regionalization of estimated mixture densities. The regionalization scheme investigated here utilizes land cover and vegetation fraction as discriminatory variables