Elsevier

Agricultural Water Management

Volume 163, 1 January 2016, Pages 75-89
Agricultural Water Management

Behavior of water balance components at sites with shallow groundwater tables: Possibilities and limitations of their simulation using different ways to control weighable groundwater lysimeters

https://doi.org/10.1016/j.agwat.2015.09.005Get rights and content

Highlights

  • Lower boundary conditions of lysimeters affect their water balance simulation.

  • Diurnal behavior cannot be reproduced properly by lysimeters using Mariotte bottles.

  • Automatic systems are more suitable for simulating natural water balance behavior.

  • Using groundwater levels as control values requires conformity with reference site.

  • The range of applications is greatly extended by lysimeters with in/outflow control.

Abstract

The water cycle of sites with shallow groundwater tables is characterized by complex interactions of hydrological and ecological processes. The water balance components, which are subject to diurnal fluctuations, are best measured with groundwater lysimeters. However, the lower boundary condition of such lysimeters affects most of the hydrological variables, particularly when considering short time scales, and has to be defined in such a way as to facilitate realistic simulations. In this paper, different means of controlling the lower boundary condition of groundwater lysimeters were compared with respect to their ability to simulate the behavior of the water balance components properly. Measurements of rain-free periods from a lysimeter station installed in the Spreewald wetland in north–east Germany were evaluated. The most common groundwater lysimeter type is controlled using a Mariotte bottle and sets the groundwater level in the soil monolith to a constant level, which here caused an alteration of the inflow to the lysimeter, with respect to both its value and diurnal behavior. Still, daily evapotranspiration values were realistic and this simple and robust approach may be used for time intervals not shorter than one day. High-resolution measurements can be gained from lysimeters that automatically adjust the groundwater level by a system of pumps and valves on an hourly basis. Still, reliable results were only obtained when the conditions in the lysimeter and the surrounding field, where the target groundwater level was measured, were in accordance. Otherwise (e.g., when the groundwater level differed) an unrealistic inflow behavior evolved. Reasonable results, even for slightly diverging conditions, were gained with a new approach that defined the lower boundary conditions by controlling the inflows and outflows of the lysimeter. This approach further enabled the groundwater level itself to be the study subject, thereby enlarging the field of possible applications of groundwater lysimeters.

Introduction

Large parts of lowlands are characterized by shallow groundwater levels, typically ranging from a few decimeters below the ground to (temporary) inundations. Tillage, crop yields and water consumption of the sites are affected by the groundwater levels, which therefore influence the potential to use the land for agriculture. The groundwater levels themselves are determined by the site’s water cycle, which is also a key driver of many biological and chemical processes.

Hydrological and ecosystem processes closely interact and are characterized by a high degree of complexity at sites with shallow groundwater tables (Laio et al., 2009). Relationships between the groundwater level and water balance components are also rather complex and cannot be viewed in isolation. Nonetheless, the water balance of such sites can be described as a classic hydrological system if the water budget is set up for a soil column.ΔS=PETa+RinRoutwhere ΔS is the water storage change, P the precipitation, ETa the actual evapotranspiration, Rin the inflow to and Rout the outflow from the soil column (Fig 1). The storage change, when not restricted to the unsaturated zone, causes a change in the groundwater level (Δgwl), whose extent depends on the storage characteristic of the soil column and the surface characteristic, when the water level is above ground.

Changes to the groundwater table subsequently can affect the gradient between the groundwater and ditch water level (Δgwd). The gradient determines whether a field is sub-irrigated, i.e., when groundwater levels are below ditch water levels (Fig. 1, right), or drained, i.e., when the groundwater table surpasses the ditch water level (Fig. 1, left). In consequence of heavy rainfall events, the groundwater level often quickly rises, which may invert the direction of flow. Instead of being sub-irrigated the field is then drained by the ditches or, if drainage conditions already prevailed before the rainfall started, the extent of drainage increases (Evans et al., 1999, Holden and Burt, 2003, Holden et al., 2006).

The groundwater level has a pronounced impact on evapotranspiration and the soil water content in the unsaturated zone. Along with that, it determines the amount of water that is available for plants and, in the long term, the composition of the site’s plant community (Booth and Loheide, 2012, Lowry et al., 2011, Muneepeerakul et al., 2008, Orellana et al., 2012, Tamea et al., 2009). Hence, the processes involved are not purely hydrological but also include biological and ecological ones, even though the temporal scales on which they operate may differ.

None of the described relationships are isolated; instead, they interact (see Eq. (1)) and incorporate feedback. Various factors affect the crop water use from shallow groundwater (as shown in a review for different agricultural crops by Ayars et al., 2006). The amount of evapotranspiration is often needed to control irrigation systems at shallow groundwater sites efficiently, as it determines the water consumption (Akhtar et al., 2013, Ayars et al., 1999, Karimov et al., 2014, Yang et al., 2007) or the drainage volume (Hornbuckle et al., 2005, Noory and Liaghat, 2009). Ultimately, the yield of agricultural crops depends on the groundwater regime (Liu and Luo, 2011, Mueller et al., 2005, Noory et al., 2009).

The mentioned studies analyze the water balance at shallow groundwater sites with different experimental methods or hydrological models, but only temporal resolutions of one day or longer were used. Thus, diurnal variations observed for some of the components at such sites were not taken into account. These variations result from interactions of meteorological and hydrological factors. The evapotranspiration shows a pronounced diurnal variation, which follows the net radiation and peaks at noon (Assouline et al., 2008, Drexler et al., 2008). The inflow to a soil column is relatively constant during rain-free days (Fahle and Dietrich, 2014), but may be subject to variation of the hydraulic gradient between the groundwater and ditch water level. The groundwater level itself, being indicative of changes in the water storage and the sum of all water balance components, also shows typical diurnal fluctuations. Its amplitude is also influenced by the soil characteristics, especially the specific yield (Acharya et al., 2012, Loheide et al., 2005, Nachabe et al., 2004).

Intra-day variations in the water balance components should be considered when studying the site’s hydrological processes in detail; however, their measurement is demanding and conducted mostly for individual components. A simple alternative is to use the diurnal fluctuations of the easily measurable groundwater level to estimate evapotranspiration and recharge. White (1932) proposed a method that yielded daily values, but newer approaches provide estimates of the water balance components at higher time resolution (Gribovszki et al., 2008, Loheide et al., 2005). The accuracy of these estimates, which can only be gained during rain-free periods, is limited (Fahle and Dietrich, 2014) due to the numerous assumptions being made. Yet exact values of the water balance variables are an important prerequisite to understand the relevant processes and form the basis for developing and applying hydrological approaches and models at such sites.

Lysimeters are able to measure all water balance components at once, even at high temporal resolution. Moreover, they can be used to analyze different variants of vegetation or hydrological regimes with respect to water consumption, crop yield or nutrient discharge. Processes in the unsaturated zone can be investigated as well (Abbasi et al., 2012). Still, lysimeters only model the natural processes and are subject to certain limitations that have to be considered. The most commonly discussed problems of lysimeters are the oasis effect, which can be minimized by installing the lysimeters directly on the corresponding investigation site, and the transferability of the lysimeter’s point measurement to a larger area (Allen et al., 2011). The latter is especially problematic at sites with very heterogeneous conditions (Alfieri et al., 2012), but is barely of importance at sites where soil, vegetation and relief conditions are homogeneous. Furthermore, the problem of transfer ability is of little importance when investigating different variants by comparing the results of different lysimeters at one station (Bethune et al., 2008, Mueller et al., 2005, Noory et al., 2009, Zhang et al., 1999).

Today, weighable lysimeters are state of the art and widely used (Lorite et al., 2012, Marek et al., 2006, Meissner et al., 2008, Von Unold and Fank, 2008). Meissner et al. (2008) described and discussed various lysimeter types (including non-weighable lysimeters) in detail. For shallow groundwater conditions, special lysimeters, so-called groundwater lysimeters, were developed. Different systems can be used to control the lower boundary condition in these lysimeters. As the present study focused on the different lysimeter control systems, the existing systems are explained more in detail in a separated Section 2.1.

The primary problem of investigations using groundwater lysimeters is the need to predetermine the lower boundary condition, i.e., the progress of the groundwater level. The groundwater table is influenced by many hydrological processes and depends on vertical and lateral flows. The lateral flows are determined by hydrological and hydraulic processes whose scale by far exceeds the scale of the soil monolith. A groundwater lysimeter therefore cannot simulate all processes adequately and the occurring deviations between natural and model conditions potentially lead to a distorted impression of the water balance.

In this paper, we demonstrate how the setting of the lower boundary conditions of groundwater lysimeters influences the water balance components. We investigated two currently used approaches which predetermine the groundwater level, and one newly developed approach which controls the lower boundary condition by specifying the in- and outflows to and from the lysimeter. An installed groundwater lysimeter station was used to simulate the three different types of groundwater control and to examine their effects on the behavior of the water balance components, the groundwater level, the soil water tension and the soil water content of the unsaturated zone. Based on our measurements, fields of application are discussed, as well as the opportunities and limitations of the individual approaches.

Section snippets

Established types of groundwater lysimeters

Two basic methods can be distinguished for controlling the groundwater level in lysimeters. In one method, the principle of the Mariotte bottle is used to achieve constant groundwater levels. In the other method, the groundwater level is controlled by adjusting the water level in a connected compensation tank using pumps and valves.

The simplest groundwater lysimeters use a Mariotte bottle, through whose air inlet the groundwater level in a connected lysimeter can be defined as the two devices

Study site and lysimeter station

The investigated site is located in the Spreewald region, about 70 km south–east of Berlin, Germany (51°52′N, 14°02′E, Fig. 3). The degraded fen is used as an extensive grassland. The soils are classified as mollic gleysoil (GIN) (FAO-ISRIC, 1990). Four horizons are designated as

  • 1)

    0–25 cm: Humic A horizon from fluvial sands (Ae(Hv)), in this case degraded fen.

  • 2)

    25–70 cm: Gley horizon made of fluvial loam including clayey and sandy portions (Bg).

  • 3)

    70–120 cm: Gley horizon including clayey and sandy

Behavior of the water balance components of the reference site

Diurnal fluctuations were observed for the field measurements of the potential evapotranspiration, groundwater level and soil water tension under rain-free conditions (Fig. 6, upper panel). The potential evapotranspiration, being closely related to the solar radiation, reached its maximum around noon and was close or below zero during the night. The actual evapotranspiration, which was not measured, can be assumed to have followed similar patterns, since the groundwater level, which reflected

Discussion

The measurements of the water balance components under rain-free conditions featured the diurnal variations that are typical of shallow groundwater sites. The interaction of all components led to an intra-day emptying and re-filling of the site’s water storage, which was reflected by the sinusoidal course of the groundwater level and variations in the unsaturated zone, which became apparent from the measured soil water tension. The amplitude of these fluctuations depended on the water balance

Conclusions

In the present study, our aim was to reveal if and how the simulation of the water cycle in a groundwater lysimeter is influenced by the specification of the lysimeter’s lower boundary condition. Our results clearly showed that such an impact existed.

A control corresponding to the use of a Mariotte bottle, which produced a nearly constant groundwater level, falsified the in- and outflows and the storage change, with respect to both their absolute values and their diurnal patterns. The

Acknowledgements

We would like to thank the German Federal Ministry of Education and Science for sponsoring the INKA BB research project (FKZ: 01LR0803A), the Authority of Environment, Health and Consumer Protection of the Federal State of Brandenburg for sponsoring part of the lysimeter station and the local water board “Oberland Calau” for supporting our work. The authors are grateful to Ute Appel and Ralph Tauschke for their field work, Evelyn Wallor for the soil evaluation and Tobias Hohenbrink for his

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