H.A.J. van Lanen

The effect of bypass flow and internal catchment of rain on the water regime in a clay loam grassland soil

Abstract Bypass flow was studied in a clay loam grassland soil with a loamy subsoil by means of laboratory experiments on large, undisturbed columns of surface soil. At a pressure head ( h ) of − 1000cm, bypass flow averaged 45% and at h = − 200 cm 70% of applied rain (intensities of 20 and 35 mm h −1 ). Depth of infiltration of bypass water was studied in the field using morphological staining techniques and an infiltration experiment. Water, flowing into continuous cracks and worm channels, infiltrated into the subsoil at 60 and 135cm depth respectively. This subsoil infiltration was called “internal catchment”. Thus, the infiltration process differs from the classical concept of surface infiltration. Simulation with 1986 weather data was used to explore the effects on the water regime. Results indicate that crop water deficits differ significantly when bypass flow and internal catchment are taken into account.

How climate seasonality modifies drought duration and deficit

Drought propagation through the terrestrial hydrological cycle is associated with a change in drought characteristics (duration and deficit), moving from precipitation via soil moisture to discharge. Here we investigate climate controls on drought propagation with a modeling experiment in 1271 virtual catchments that differ only in climate type. For these virtual catchments we studied the bivariate distribution of drought duration and standardized deficit for the variables precipitation, soil moisture, and discharge. We found that for meteorological drought (below-normal precipitation), the bivariate distributions of drought characteristics have a linear shape in all climates and are thus not affected by seasonality in climate. Despite the linear shape of meteorological drought, soil moisture drought (below-normal storage in the unsaturated zone) and hydrological drought (below-normal water availability in aquifers, lakes, and/or streams) show strongly nonlinear shapes in drought characteristics in climates with a pronounced seasonal cycle in precipitation and/or temperature. These seasonality effects on drought propagation are found in monsoonal, savannah, and Mediterranean climate zones. In these regions, both soil moisture and discharge show deviating shapes in drought characteristics. The effect of seasonality on drought propagation is even stronger in cold seasonal climates (i.e., at high latitudes and altitudes), where snow accumulation during winter prevents recovery from summer hydrological drought, and deficit increases strongly with duration. This has important implications for water resources management in seasonal climates, which cannot solely rely on meteorology-based indices as proxies for hydrological drought duration and deficit and need to include seasonal variation in both precipitation and temperature in hydrological drought forecasting.

Probabilistic analysis of hydrological drought characteristics using meteorological drought

Abstract Droughts are an inevitable consequence of climate variability and are pervasive across many regions. Their effects can vary on an extensive scale, depending on the type of drought and peoples vulnerability. Crucial characteristics of both hydrological (groundwater, streamflow) and meteorological (precipitation) droughts are related to their durations and severities, and these characteristics are typically correlated. While many studies have addressed the dependencies between these characteristics for either the meteorological or hydrological drought, the cross-dependence between meteorological and hydrological drought characteristics is barely investigated. The development of meteorological drought characteristics to hydrological drought characteristics is often hard to model and their connection is not definitively established. In order to better understand and explain this relationship, this study seeks to apply statistical tools and models. Drought characteristics data from areas in Europe with different climates are analysed. Two approaches of identifying related meteorological and hydrological drought are explored and compared. Classical linear correlation techniques do not provide promising results, indicating that any statistic of a hydrological drought is not a straightforward function of a preceding meteorological drought. Subsequently, the application of the concept of copulas to explore this dependence between meteorological and hydrological drought characteristics is investigated. The more comprehensive approach of copulas shows that the meteorological drought contains probability information of the successive hydrological drought. Editor Z.W. Kundzewicz Citation Wong, G., van Lanen, H.A.J., and Torfs, P.J.J.F., 2013. Probabilistic analysis of hydrological drought characteristics using meteorological drought. Hydrological Sciences Journal, 58 (2), 253–270.

Definition, Effects and Assessment of Groundwater Droughts

Natural groundwater droughts originate from reduced recharge over a prolonged period of time and these droughts are often enhanced by human activities (e.g. abstractions). Low groundwater heads and small groundwater gradients cause the droughts. Groundwater droughts are mainly associated with low well yields, which affect public water supply and irrigation practices and are related to other droughts (e.g. agricultural droughts). Groundwater drought monitoring and assessment methods are based upon an analysis of time-series of groundwater recharge or groundwater heads and require a threshold or probability occurrence, which can only be derived from a proper evaluation of the effects of groundwater droughts, such as lowering of well levels, reduction of springflow or reduction of capillary rise. Some of these effects are discussed to illustrate how eventually such threshold or probability of occurrence has to be determined.

Global Multimodel Analysis of Drought in Runoff for the Second Half of the Twentieth Century

During the past decades large-scale models have been developed to simulate global and continental terrestrial water cycles. It is an open question whether these models are suitable to capture hydrological drought, in terms of runoff, on a global scale. A multimodel ensemble analysis was carried outtoevaluate if 10 such large-scale models agree on major drought events during the second half of the twentieth century. Time series of monthly precipitation, monthly total runofffrom 10 global hydrological models, and their ensemble median have been used to identify drought. Temporal development of area in drought for various regions across the globe was investigated. Model spread was largest in regions with low runoff and smallest in regions with high runoff. In vast regions, correlation between runoff drought derived from the models and meteorological drought was found to be low. This indicated that models add information to the signal derived from precipitation and that runoff drought cannot directly be determined from precipitation data alone in global drought analyses with a constant aggregation period. However, duration and spatial extent of major drought events differed between models. Some models showed a fast runoff response to rainfall, which led to deviations from reported drought events in slowly responding hydrological systems. By using an ensemble of models, this fast runoff response was partly overcome and delay in drought propagating from meteorological drought to drought in runoff was included. Finally, an ensemble of models also allows for consideration of uncertainty associated with individual model structures.

Water flow and nitrate transport to a groundwater‐fed stream in the Belgian–Dutch chalk region

Human activities, such as high fertilizer applications, groundwater abstractions and land use changes, might have a negative impact on drinking water supply, environment and nature reserves of the chalk region in the Dutch–Belgian boundary area. In this area, the groundwater and surface water systems of the Noor experimental catchment have been monitored intensively and modelled to investigate groundwater flow and the transport of nitrate to groundwater-dominated streams. Groundwater flow towards the riparian area, the springs and Noor brook is strongly controlled by long-term variation of the groundwater recharge. Relatively small, but permanent changes in the recharge have a higher impact on spring flow and stream flow than current groundwater abstraction in this chalk catchment. Land use, groundwater flow patterns and the characteristics of the geological formations result in distinctly different groundwater types and associated nitrate concentrations. High concentrations were found under the plateau (median 55 mg · l−1) and extremely low in the riparian area (median 2 mg · l−1). The springs and thereby the Noor brook are mainly fed by groundwater directly coming from the plateau with a low probability of denitrification. This results in median NOconcentrations which are clearly above the drinking water standard of 50 mg · l−1 (i.e. 68 and 58 mg · l−1). Monitoring and modelling show that the north springs have higher NOconcentrations than the south springs due to different probabilities of denitrification. Nitrate in the groundwater-dominated Noor brook is hardly correlated with the discharge, but the major spring shows a clear upward trend in terms of nitrate concentrations, i.e. an increase by about 30 mg · l−1 from 1980 onwards. A policy of reduced nitrogen application will result in lower nitrate concentrations in the surface water system not earlier than in 10 or 20 years, because of the long travel times of water particles in the unsaturated and saturated zone. The north springs will react earlier than the south springs. Copyright