Peter Troch

Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought

Large-scale biogeographical shifts in vegetation are predicted in response to the altered precipitation and temperature regimes associated with global climate change. Vegetation shifts have profound ecological impacts and are an important climate-ecosystem feedback through their alteration of carbon, water, and energy exchanges of the land surface. Of particular concern is the potential for warmer temperatures to compound the effects of increasingly severe droughts by triggering widespread vegetation shifts via woody plant mortality. The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates—temperature-sensitive carbon starvation in response to a period of protracted water stress or temperature-insensitive sudden hydraulic failure under extreme water stress (cavitation). Here we show that experimentally induced warmer temperatures (≈4 °C) shortened the time to drought-induced mortality in Pinus edulis (piñon shortened pine) trees by nearly a third, with temperature-dependent differences in cumulative respiration costs implicating carbon starvation as the primary mechanism of mortality. Extrapolating this temperature effect to the historic frequency of water deficit in the southwestern United States predicts a 5-fold increase in the frequency of regional-scale tree die-off events for this species due to temperature alone. Projected increases in drought frequency due to changes in precipitation and increases in stress from biotic agents (e.g., bark beetles) would further exacerbate mortality. Our results demonstrate the mechanism by which warmer temperatures have exacerbated recent regional die-off events and background mortality rates. Because of pervasive projected increases in temperature, our results portend widespread increases in the extent and frequency of vegetation die-off.

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

Abstract 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. Koutsoyiannis Citation Hrachowitz, M., Savenije, H.H.G., Blöschl, 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 decade of Predictions in Ungauged Basins (PUB)—a review. Hydrological Sciences Journal, 58 (6), 1198–1255.

The future of hydrology: An evolving science for a changing world

Human activities exert global-scale impacts on our environment with significant implications for freshwater-driven services and hazards for humans and nature. Our approach to the science of hydrology needs to significantly change so that we can understand and predict these implications. Such an adjustment is a necessary prerequisite for the development of sustainable water resource management strategies and to achieve long-term water security for people and the environment. Hydrology requires a paradigm shift in which predictions of system behavior that are beyond the range of previously observed variability or that result from significant alterations of physical (structural) system characteristics become the new norm. To achieve this shift, hydrologists must become both synthesists, observing and analyzing the system as a holistic entity, and analysts, understanding the functioning of individual system components, while operating firmly within a well-designed hypothesis testing framework. Cross-disciplinary integration must become a primary characteristic of hydrologic research, catalyzing new research and nurturing new educational models. The test of our quantitative understanding across atmosphere, hydrosphere, lithosphere, biosphere, and anthroposphere will necessarily lie in new approaches to benchmark our ability to predict the regional hydrologic and connected implications of environmental change. To address these challenges and to serve as a catalyst to bring about the necessary changes to hydrologic science, we call for a long-term initiative to address the regional implications of environmental change.

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.

Effective water table depth to describe initial conditions prior to storm rainfall in humid regions

A physically meaningful technique to determine the effective depth to the water table, as a measure of the initial storage capacity of a basin is developed. The estimation of the initial storage capacity prior to a given flood event is essential to obtain useful results from storm runoff prediction models based on saturation excess overland flow. It is shown how this effective depth to the water table can be related to streamflow measurements at the outlet of the basin. The analysis is based on Boussinesqs standard hydraulic groundwater theory. The main feature of the present formulation is that it allows the estimation of catchment-scale parameters, namely the aquifer hydraulic conductivity and the average depth to the impervious layer. The estimation of these parameters is based on a drought flow analysis which is consistent with the hydraulic groundwater theory used to develop the described technique. This hydraulic theory is found to be applicable for a catchment under humid temperate climatic conditions, namely the Zwalm catchment situated in East-Flanders, Belgium. The results of the proposed analysis are used to estimate the initial conditions in a partial area runoff generation model. It is shown that accurate estimates of total runoff volume are obtained without further calibration of the model.

Comparative hydrology across AmeriFlux sites: The variable roles of climate, vegetation, and groundwater

Watersheds can be characterized as complex space?time filters that transform incoming fluxes of energy, water, and nutrients into variable output signals. The behavior of these filters is driven by climate, geomorphology, and ecology and, accordingly, varies from site to site. We investigated this variation by exploring the behavior of evapotranspiration signals from 14 different AmeriFlux sites. Evapotranspiration is driven by water and energetic forcing and is mediated by ecology and internal redistribution of water and energy. As such, it integrates biological and physical controls, making it an ideal signature to target when investigating watershed filtering. We adopted a paradigmatic approach (referred to as the null model) that couples the Penman?Monteith equation to a soil moisture model and explored the deviations between the predictions of the null model and the observed AmeriFlux data across the sites in order to identify the controls on these deviations and their commonalities and differences across the sites. The null model reproduced evapotranspiration fluxes reasonably well for arid, shallow?rooted systems but overestimated the effects of water limitation and could not reproduce seasonal variation in evapotranspiration at other sites. Accounting for plant access to groundwater (or deep soil moisture) reserves and for the effects of soil temperature on limiting evapotranspiration resolved these discrepancies and greatly improved prediction of evapotranspiration at multiple time scales. The results indicate that site?specific hydrology and climatic factors pose important controls on biosphere?hydrosphere interactions and suggest that plant–water table interactions and early season phenological controls need to be incorporated into even simple models to reproduce the seasonality in evapotranspiration.

Retrieving Soil Moisture Over Bare Soil from ERS 1 Synthetic Aperture Radar Data: Sensitivity Analysis Based on a Theoretical Surface Scattering Model and Field Data

In order to assess the retrieval of soil moisture from ERS 1 (European Remote Sensing Satellite) synthetic aperture radar (SAR) data, an inversion procedure based oft the integral equation model (IEM) [Fung et al., 1992] is developed. First, the IEM is used to analyze the sensitivity of radar echoes (in terms of the backscattering coefficient σ0) to the surface parameters (roughness and dielectric constant) under ERS 1 SAR configuration. Results obtained for random rough bare soil fields show that the effect of surface roughness is very strong, particularly in the case of smooth surfaces, and that the sensitivity of σ0 to dielectric constant is independent of the radar configuration and the roughness conditions. This means that the range of variation of backscattering with respect to the dielectric constant variation of dry to wet soil remains the same (about 5 dB) for any roughness condition and radar configuration. The possibility of applying the inversion procedure to retrieve soil moisture is investigated using a set of data collected in a test site situated near Naples, Italy, during the SeIe Synthetic Aperture Radar experiment (SESAR) campaign (November 1993). Simultaneous with ERS 1 overpasses, dielectric constant and roughness measurements were taken over two flat bare fields. From this analysis it is found that the inversion of backscattering from ERS 1 SAR into soil moisture is not reliable without accurate information on roughness if the surface is smooth. In this case it is observed that the sensitivity to the roughness parameters is much higher than the sensitivity to dielectric constant, so that even a small error in the measurement of this parameter can affect the retrieved value of soil moisture significantly. The inversion procedure provides more reliable soil moisture estimates when surfaces rougher than those analyzed in the field experiment are considered.

Assimilation of active microwave observation data for soil moisture profile estimation.

This paper discusses the potential of retrieving information about the soil moisture profile from measurements of the surface soil moisture content through active microwave observations of the Earth. Recently, Mancini et al. [1999] have shown through laboratory experiments that the volumetric moisture content of the first few centimeters of a bare soil can be determined within 5 ol accuracy by means of C and L band active microwave observations and inverse modeling. Here we use active microwave observations of the surface soil moisture content in a data assimilation framework to show that this allows the retrieval of the root zone soil moisture profile. The data assimilation procedure developed is based on the Kalman filter technique. Kalman filtering allows reconstruction of the state vector of a system when this system is represented by a dynamic model and when at least part of the state variables are observed regularly. The dynamic model of the system used here is based on the one-dimensional Richards equation. The observation equation is based on the Integral Equation Model [Fung et al., 1992; Fung, 1994] and is used to link the radar observations to surface soil moisture content. It is shown that even in the presence of model and observation noise and infrequent observations, accurate retrieval of the entire moisture profile is possible for a bare soil. ? 2000 American Geophysical Union

Some analytical solutions of the linearized Boussinesq equation with recharge for a sloping aquifer

Subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the Boussinesq equation. Several attempts have been made to solve this partial differential equation, and exact solutions have been found for specific situations. In the case of a sloping aquifer, Brutsaert [1994] suggested linearizing the equation to calculate the unit response of the hillslope. In this paper we first apply the work of Brutsaert by assuming a constant recharge to the groundwater table. The solution describes the groundwater table levels and the outflow in function of time. Then, an analytical expression is derived for the steady state solution by allowing time to approach infinity. This steady state water table is used as an initial condition to derive another analytical solution of the Boussinesq equation. This can then be used in a quasi steady state approach to compute outflow under changing recharge conditions. | Subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the Boussinesq equation. Several attempts have been made to solve this partial differential equation, and exact solutions have been found for specific situations. In the case of a sloping aquifer, Brutsaert [1994] suggested linearizing the equation to calculate the unit response of the hillslope. In this paper we first apply the work of Brutsaert by assuming a constant recharge to the groundwater table. The solution describes the groundwater table levels and the outflow in function of time. Then, an analytical expression is derived for the steady state solution by allowing time to approach infinity. This steady state water table is used as an initial condition to derive another analytical solution of the Boussinesq equation. This can then be used in a quasi steady state approach to compute outflow under changing recharge conditions.

Analytical solutions to a hillslope-storage kinematic wave equation for subsurface flow

Abstract Hillslope response has traditionally been studied by means of the hydraulic groundwater theory. Subsurface flow from a one-dimensional hillslope with a sloping aquifer can be described by the Boussinesq equation [Mem. Acad. Sci. Inst. Fr. 23 (1) (1877) 252–260]. Analytical solutions to Boussinesqs equation are very useful to understand the dynamics of subsurface flow processes along a hillslope. In order to extend our understanding of hillslope functioning, however, simple models that nonetheless account for the three-dimensional soil mantle in which the flow processes take place are needed. This three-dimensional soil mantle can be described by its plan shape and by the profile curvatures of terrain and bedrock. This plan shape and profile curvature are dominant topographic controls on flow processes along hillslopes. Fan and Bras [Water Resour. Res. 34 (4) (1998) 921–927] proposed a method to map the three-dimensional soil mantle into a one-dimensional storage capacity function. Continuity and a kinematic form of Darcys law lead to quasi-linear wave equations for subsurface flow solvable with the method of characteristics. Adopting a power function of the form proposed by Stefano et al. [Water Resour. Res. 36 (2) (2000) 607–617] to describe the bedrock slope, we derive more general solutions to the hillslope-storage kinematic wave equation for subsurface flow, applicable to a wide range of complex hillslopes. Characteristic drainage response functions for nine distinct hillslope types are computed. These nine hillslope types are obtained by combining three plan curvatures (converging, uniform, diverging) with three bedrock profile curvatures (concave, straight, convex). We demonstrate that these nine hillslopes show quite different dynamic behaviour during free drainage and rainfall recharge events.