Potential flow theory, Dupuit-Forchheimer theory, or a combination of both is used for the derivation of drainage equations. In some cases empirical relations, with some physical justification, also have been proposed. This chapter discusses drainage equations for steady flow to drains in homogeneous and layered soils, as well as steady flow to wells. The necessity of soil drainage is not restricted to flat areas. Many areas with sloping land require drainage as well. The development of drain-spacing equations or efficient drain systems for such conditions is more complicated than in the case of flat land. The work of Glover shows that interceptor drains can be highly effective in lowering the water table on hillslopes. However, they can be equally effective in flat areas at the foot of hillslopes, as an analysis of Kirkham and Prunty or of Morin and Warrick has shown.

An outline of Hooghoudt's early career is followed by a detailed discussion of his contributions to the improvement of methods for the physical characterization of soils, and to the development of the theory of drainage. The early motivation came from the need for a rational approach to drainage of the newly reclaimed soils of the IJsselmeer polders. Wide application in development of sport fields, parks, and airfields soon followed. Hooghoudt was also a pioneer of regional hydrological studies. In the 1950s, Dutch-born PhD students at Iowa State University and Cornell University effectively promoted Hooghoudt's ideas, published originally mainly in Dutch.

Flooding on the German Rhine during the 20th century was tested for trends and assessed to identify causal mechanisms driving worsening of flooding. A review of previous research outlines the range of impacts due to climate change, land-use shifts, and river regulation. Analysis of hydrologic data, especially of the long record at Cologne, documents statistically significant increases in both flood magnitudes and frequencies. Specific-gauge analysis, which isolates the effects of channel modification, documents that 20th century river engineering has caused little of the observed increase in flooding on the German Rhine. Precipitation records from the Rhine basin confirm that flood magnification has been driven by upstream factors, including an increase in flood-producing precipitation of roughly 25% during the past 100 years and increases in runoff yields. In addition, agricultural land-use records suggest that flood magnification can be partially explained by 20th century trends documenting intensification and industrialization of German agriculture. Copyright (c) 2005 John Wiley & Sons, Ltd.

SummaryWater repellency can be a significant factor in soil physical behaviour, but little is known about the depth dependence of the contact angle of field soils. We investigated contact angles and wetting properties as a function of depth for a wide range of agricultural and forest soils in Germany. The agricultural soils ranged from silty to sandy texture (six profiles), and the forest soils ranged from sandy to loamy texture (eight profiles). Contact angles (CA) were measured with the Wilhelmy plate method (WPM). In most of the soils, advancing WPM contact angles were considerably greater than 0° and they varied irregularly with depth. In general, sandy soils had larger WPM contact angles than silty soils. From the relation of the contact angle with texture and pH the quality of soil organic matter (SOM) was considered as more important for the wetting properties than the total amount of soil organic carbon (SOC). Finally, it was found that for soils with intermediate sand contents either under agricultural or forest use, the kind of land use seemed not to influence CA. Coarse‐textured sandy soils that were used only as forest sites were more hydrophobic than silty soils which were exclusively used as agricultural soils. We conclude that a coarse texture favours, in combination with other factors (mainly pH), hydrophobic SOM.

A 34-year record of dredging in a 484-km reach of the Middle and Upper Mississippi River documents the spatial and temporal patterns of bed aggradation in an intensively engineered river. Between 1964 and 1997, 183 millionm3 was removed from the study reach, with 112.6 km of the channel undergoing dredging of some kind, 12.1 km requiring ≥5 dredgings during this period, 2.6 km requiring ≥10 dredgings, and one site requiring 29 dredgings in 34 years. Forty-three sites were identified where dredging volume was exceptionally high and/or were frequently repeated. These sites occur in five settings: (1) where flow is divided through chutes or side channels; (2) at the mouths of largely unregulated tributary streams; (3) at thalweg crossings in meander bends; (4) in long, straight reaches of the channel; and (5) near problematic engineered structures, the principal one in the study area being the outlet of the Chain of Rocks Canal. In the pooled reach of the Upper Mississippi, impoundment has had a major influence on shoaling, with 80% of dredged volume required in the upstream halves of the pools. Although past engineering modifications have reduced sediment aggradation in the Mississippi channel, structural solutions to continuing shoaling can adversely impact the river's ecology, flood response, or hydrological function, and many structural solutions appear to be approaching their effective limits. The exception to this may be at highly localized shoaling locations, such as the Chain of Rocks Canal outlet, where flaws in the original design may still be remedied by structural means Targeted structural solutions may be effective at solving limited local shoaling problems, but we suggest that more sweeping new engineering modifications to reduce or eliminate dredging requirements are likely to be ineffective and/or outweighed by their adverse effects on the river system.

To control the groundwater contamination by NO3, monitoring programs have been developed during the past 17 years in German drinking water catchments. The monitoring programs are using data of the NO3 content of the soils and of the groundwater. In Baden-Wurttemberg, between 1987 to 2002, soil NO3 decreased significantly by 50%. Atmospheric influences and irregularities of sampling complicate the interpretation of NO3 values in the soil. This is especially true for areas, where values are available only for about one decade or less. To take such complications into account, NO3 time series from a drinking water catchment in Lower Saxony(Germany) were analyzed by multiple regression. A strong correlation between the weather conditions (precipitation and temperature during the fall) and the NO3 content of the soil was found While the raw data of NO3 content of the soil did not decrease significantly over time, a highly significant yearly decrease of about 3kg N (.) ha(-1) or 50% from 1.992 to 2002 was observed after atmospheric influences were taken into account. This result was confirmed by the trend of NO3 concentration of the groundwater near the surface. It decreased yearly by about 10mg (.) L-1 or 55% from 1994 to 2001. It is concluded that a multiple regression analysis is indispensable in interpreting soil NO, time series of restricted length.

To control the groundwater pollution by NO3-, different monitoring programs have been carried out during more than a decade in drinking water catchments in Germany. The monitoring programs are based on agricultural and hydrological data. Atmospheric influences and irregularities of sampling complicate the interpretation of NO3- values in the soil and in the groundwater. To take such complications into account, NO3- time series from a drinking water catchment in Lower Saxony were analyzed by multiple regression. A strong correlation between the precipitation during the fall season and the NO3- content of the soil was found. The NO3- content of the soil is also influenced by the October temperature. While the raw data of NO3- content of the soil did not decrease significantly over time, a highly significant yearly decrease of about 3 kg N ha(-1) or 50 % from 1992 to 2002 was observed after atmospheric influences were taken into account. The variability due to precipitation was amplified by changing the date at which samples were taken. The NO3- concentration of the groundwater near the surface decreased yearly by about 10 mg 1(-1) or 55 % from 1993 to 2001. Beside this decrease, influences of the October temperature and the sampling depth on the NO3- concentration of groundwater were identified. It is concluded that a multiple regression analysis is indispensable in interpreting soil and groundwater NO3- time series of restricted length.

Physical degradation of agricultural soils is a widespread and persistent problem, and it is a problem that is now increasingly recognized in Germany. It causes damages and external costs that are estimated at several hundred million Euros per year. If, and to what extent, Tschernosem cropland soils of the so-called Hildesheimer Borde (Hildesheim Loess Belt) are subject to physical soil degradation has been examined little to date. Therefore we studied the deterioration of soil structure in the plow layer as well as soil compaction below the plow pan at a number of arable field locations in the Borde region. As a measure of soil structure, we chose the aggregate stability, and for soil compaction we chose the relative Proctor density. In order to assess the development of soil degradation over time, we compared our present measurements with those from the same sites made during the 1960s. We found that already in the 1960s, evidence of physical soil degradation could be detected in the cropland soils of the Hildesheimer Borde. This tendency, however, increased in the following decades. The relative aggregate stability of the plow layer presently amounts to only about 10%, and the soil density below the plow pan shows values of about 95% of the Proctor density. To protect the soil and the environment, measures against further degradation promoting reestablishing of lost functionalities of the Borde soils appear appropriate. The relative aggregate stability (qGMD) seems to be a proper criterion for imposing the new Federal Soil Protection Act. Furthermore, reduced soil cultivation, together with a restriction in the size and power of farm machinery, would be helpful. A restriction in the size and the length of single fields might also be considered. We show that such restrictions may lead to farm income losses, but because such restrictions are economically and environmentally beneficial, they could be compensated by diverting existing financial farm subsidies.

In many parts of the world the frequency of river floods (flash floods) seems to have increased during the past half century. Intensified agriculture is considered as a possible cause for the changed peak flow behavior. It is believed that a large-scale, narrowly designed subsurface drainage reduces the soil water retention in periods with excessive precipitation or snow melt. To increase the soil water retention, it may be necessary to reconsider conventional drain spacing design. The present study deals with the calculation of drain spacings for optimal rainstorm runoff control. A semi-analytical procedure is developed with which for a given extreme rainfall event the drain spacing is calculated that provides the highest possible soil water retention, but no surface runoff. The model considers two-dimensional unsteady water flow between parallel tile drains, with a rising water table. It combines an analytical rising water table model with an empirical spreading water table model. A comparison of the new and a conventional drain design system (Hooghoudt–Ernst) shows that with the newly designed system a considerable temporary soil water retention during heavy rainfall can be achieved. For example, for a soil with a hydraulic conductivity of 0.5md−1 that is underlain by an impervious barrier at the 2.0m depth, and that is drained by tiles with a radius of 0.1m at the 1.0m soil depth, an additional soil water retention of 38mm is obtained when the drain spacing is 46.0m instead of 13.5m for a rainfall event of 80mm in a 4-day period. The newly proposed design system may help to reduce the flood threat in areas with large-scale agricultural drainage in periods with excessive rainfall or snow melt.

In Germany, mounds of solid wastes (mainly rock salt, NaCl) from potash mining produce large amounts of briny runoff which are frequently conveyed into surface waters. This study was conducted to evaluate a saline, fine-grained aluminum recycling by-product as soil substitute in a surface barrier over potash mining waste mounds. Four free-drainage lysimeters were monitored for three years under field conditions. Two lysimeters were filled with a by-product from aluminum recycling, and the other two with a mixture of this by-product and a coal combustion waste. Precipitation, evaporation, discharge, as well as pressure head in three depths were measured continuously. Electrical conductivity in the discharge and in suction cup solutions from four depths was used to monitor desalinization. It was found that mean annual discharge from the pure metallurgical waste and from the mixture was 39% and 24% of rainfall. Materials were sufficiently leached to support growth of ryegrass (Lolium perenne, L.) after 444 (pure metallurgical waste) and 281 mm (mix) of seepage, or 28 months of lysimeter operation. We conclude that the mix seems the better material for an engineered mine-waste surface barrier.

A three-year long lysimeter experiment with a fine-grained aluminum (Al) recycling by-product and a mixture of this by-product and a coal combustion waste was conducted. The wastes were proposed as possible soil substitutes in an engineered surface barrier covering a potash mining residue mount. To evaluate the suitability of the wastes as surface barrier material, their hydrological behavior under field conditions must be known. Lysimeter experiments provide one means to study the hydrological behavior of soils or soil-like materials. However, it is difficult to estimate the long-term hydrological behavior from short-term lysimeter studies. The present study was conducted therefore to derive from short-term lysimeter observations the long-term hydrological behavior of the two waste materials. The lysimeter data were used to calibrate the one-dimensional soil water flow model HYDRUS-1D. With the calibrated model, hydrological simulations for the site of the residue mount were carried out for a period of 31 yr. Calculated long-term annual seepage from the lysimeters was 237 mm for the pure Al waste and 186 mm for the mixture, or 39% and 24% of the average annual precipitation (764 mm). The average discharge of the bare mount is 482 mm or 63%. We conclude that a soil cover could considerably reduce the discharge and that the mixture is better suited as surface barrier than the pure Al waste.

Disposal of highly saline industrial by-products in landfills is not permitted in member states of the European Union, such as Germany. Large amounts of such by-products thus have to be disposed of in alternative ways. In many countries bare potash mining residue mounds, consisting almost entirely of rock salt (NaCl), pose environmental problems. Covering such mounds with soil or soil-like material could help to reduce the yearly amount of briny runoff. A fine-granular saline aluminum recycling by-product (ALRP) has been proposed as a soil substitute to cover rock salt residue mounds. Use of this by-product as a combined soil substitute and surface barrier is not considered to be a landfill disposal, but as a beneficial by-product reuse. To judge the feasibility of ALRP for this purpose, its properties must be known. In this study physical characteristics of an industrially produced ALRP, mixed with the flue gas desulfurization by-product (FGDP) of a coal combustion power plant, were determined. It was found that the texture of both ALRP and ALRP-FGDP mix was silt loam. Bulk densities of ALRP and ALRP-FGDP were 0.93 and 0.88 Mg x m(-3) and the corresponding salt contents were 50.0 and 35.5%, respectively. The erodibility factor K of pure ALRP was estimated as 0.65 Mg h x ha (-1)N(-1). Because of the stabilizing effect of FGDP, this factor was reduced considerably in ALRP-FGDP. The water-holding capacity of unwashed ALRP was 44.5% and of washed ALRP-FGDP 61.8%. In view of its physical properties, ALRP-FGDP seems to be suitable as an evaporation enhancing, runoff reducing cover material for potash mine residue mounds, even on steep slopes. Use of ALRP, mixed with FGDP, as a soil substitute in a surface barrier, thus seems to be environmentally meaningful. However, the high salt content initially prevents plant growth. With time, after the salt has been leached, the material seems able to support plant growth, which would further reduce runoff. The physical and hydraulic parameters determined in this study may serve future users of similar by-products.

It seems that the frequency of floods along the main rivers in Germany increased during the second half of the past century. A number of causes for this phenomenon have been suggested. We hypothesise that postwar changes in agricultural land use also play a role. For example, the meadowland area in former West Germany decreased between 1951 and 1989 from 15.7 to 10.8%. Simultaneously, the small grain acreage grew from 18.5 to 22.3%. Additionally, nearly 20% of the agricultural land area was drained artificially during this period. We used the US Soil Conservation Service rainfall-runoff model for small drainage basins to estimate the possible increase in surface runoff during heavy rainstorms because of the observed changes in agricultural land use. Our model calculations suggest that increased surface runoff during large-scale heavy rainstorms may contribute substantially to the present flood problem in Germany.

The present agricultural production system in Germany is characterized by a high degree of commercial N-fertilizer and imported fodder use. Because the N use efficiency is low, a large part of the applied N is emitted in a reactive form into the environment. The possible damage caused in the environment so far has been investigated only partly. The objective of the present study therefore was to quantifiy this environmental damage. To this end, the yearly N surplus in German agriculture since 1951 was estimated with the use of statistical data on national agriculture. With the use of literature data, we showed how, and to which extent, surplus N is emitted into the environment. We also evaluated monetarily the environmental damage. To judge the magnitude of the environmental damage, the monetary damage was compared with the economical output of agriculture. Our calculations aim at directing ways to develop a sustainable agricultural production system, to which Germany has committed itself by signing the Agenda 21.For the period under study (1951 to 2000), we calculated from the difference between the N input by commercial fertilizers and imported fodder, and the N output by animal and crop products the annual N surplus. This surplus increased from about 10 kg/ha in the 1950s to 120 kg/ha in the 1980s, and amounted to 70-80 kg/ha in the 1990s. In case additional N inputs are considered, such as N fixation by legume crops, N deposition from the atmosphere, or N input by compost and sludge, and if the lay-off cropland area is subtracted from the total agricultural land area, the calculated N surplus per hectare of cropland is considerably larger. For 1995, for example, in this case a N surplus of nearly 150 kg/ha is calculated, which is emitted largely into the environment (ground and surface waters, atmosphere). The annual external costs caused by these emissions are estimated at 2.5 to 10.0 billion DM (approximate to 1.25 to 5.0 EU$). Furthermore we point out, that particularly animal products are consumed to such an extent that it harms the population's health. Stimulating and subsidizing organic farming may help to reduce the excessive use of nitrogen in today's agriculture. It appears, that in view of the present state of science and technology, the NO3 and agricultural N surplus problem in Germany can be solved enduringly only with a significant reduction in animal production.

Study of soil physical processes such as water infiltration and redistribution, groundwater recharge, solute transport in the unsaturated zone, compaction and aeration in variably saturated soil hardly is possible without knowledge of the capillary pressure of the soil water as a function of the degree of saturation. Pore space topology, interfacial tension, and temperature probably are the most important physical factors affecting the capillary pressure at a given water content. Despite intensive research in the past decades on the water retention characteristics of soils, our knowledge of their response to varying ambient conditions is far from being complete. Current models of soil water retention as well as of hydraulic conductivity for unsaturated porous media often still use the simplified representation of the pore system as a bundle of cylindrical capillaries. Physical effects, like surface water film adsorption, capillary condensation and surface flow in liquid films, as well as volumetric changes of the pore space are often ignored. Consequently, physical properties of the solid phase surfaces, and their impact on water adsorption and flow, are often not considered. The objective of this contribution is to review various interfacial properties with possible application to the conventional water content - matric potential relation of soils. The ignoring of interfacial effects on the water retention of soils is widespread in the literature. The motivation of this paper is therefore to point out some of the more significant deficiencies of our current knowledge on the interaction of solid particle surfaces and the liquid phase in soil. We will first emphasize the impact of the wetting angle on the wetting of dry soil and to present the impact of interfacial tension of the liquid phase in the three-phase system. At low water content, the transition from capillary-bound water to adsorbed water and to wetting films is discussed separately, because of its impact on the rewetting process of dry soil. Finally, we discuss the impact of temperature on interfacial tension and water retention of soil as a second important interfacial process affecting directly the water retention of porous media.

Preferential flow in soil can enhance the leaching of agricultural chemicals. In a number of studies it has been shown that the mobile-immobile solute transport model (MIM) is a useful tool to characterize preferential flow. In the present study, a new laboratory method for determining the MIM parameters theta(m) and theta(im) (mobile and immobile water content), as well as alpha (mass transfer coefficient), is developed. The computations are uncomplicated and the method requires only simple equipment. It is applied to short, undisturbed soil columns. Measured values ranged from 0.11 to 0.27 for theta(im) theta(-1) and from 0.015 h(-1) to 0.034 h(-1) for alpha for an Iowan soil (Nicollet silt loam). For two sandy Eutric Gleysols from Germany, low values for theta(im) theta(-1) from 0.04 to 0.07 and from 0.001 h(-1) to 0.008 h(-1) for alpha were determined. Although the new method is a flow-interruption technique, values for the Nicollet silt loam compare well with those from conventional leaching experiments. Values for the Eutric Gleysols agree with the observation that these soils were poorly structured. Because the new method does not assume negligible dispersion, it is applicable to a wider range of soils and boundary conditions than comparable approaches. We conclude that the new method provides parameter values that are suited to describe non-equilibrium solute transport.

The capillary pressure (ψ) in unsaturated porous media is known to be a function of temperature (T). Temperature affects the surface tension (σ) of the pore water, but possibly also the angle of contact (γ). Because information on the temperature dependence of γ in porous media is rare, we conducted experiments with three wettable soils and their hydrophobic counterparts. The objectives were (i) to determine the temperature dependence of the water retention curve (WRC) for wettable and water‐repellent soils, (ii) to assess temperature effects on the apparent contact angle γA derived from those WRCs, and (iii) to evaluate two models (Philip‐de Vries and Grant‐Salehzadeh) that describe temperature effects on ψ. Columns packed with natural or hydrophobized soil materials were first water saturated, then drained at 5, 20, and 38°C, and rewetted again to saturation. Capillary pressure and water content, θ, at five depths in the columns were measured continuously. The observations were used to determine the change in γA with T, as well as a parameter β0 that describes the change in ψ with T It was found that the Philip‐de Vries model did not adequately describe the observed relation between ψ and T A mean value for β0 of −457 K was measured, whereas the Philip‐de Vries model predicts a value of −766 K. Our results seem to confirm the Grant‐Salezahdeh model that predicts a temperature effect on γA For the sand and the silt we studied, we found a decrease in γA between 1.0 to 8.5°, when the temperature was increased from 5 to 38°C. Both β0 and γA were only weak functions of θ. Furthermore, it seemed that for the humic soil under study, surfactants, i.e., the dissolution of soil organic matter, may compound the contact angle effect of the soil solids.