@article{FahleHohenbrinkDietrichetal.2015, author = {Fahle, Marcus and Hohenbrink, Tobias Ludwig and Dietrich, Ottfried and Lischeid, Gunnar}, title = {Temporal variability of the optimal monitoring setup assessed using information theory}, series = {Water resources research}, volume = {51}, journal = {Water resources research}, number = {9}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0043-1397}, doi = {10.1002/2015WR017137}, pages = {7723 -- 7743}, year = {2015}, abstract = {Hydrology is rich in methods that use information theory to evaluate monitoring networks. Yet in most existing studies, only the available data set as a whole is used, which neglects the intraannual variability of the hydrological system. In this paper, we demonstrate how this variability can be considered by extending monitoring evaluation to subsets of the available data. Therefore, we separately evaluated time windows of fixed length, which were shifted through the data set, and successively extended time windows. We used basic information theory measures and a greedy ranking algorithm based on the criterion of maximum information/minimum redundancy. The network investigated monitored surface and groundwater levels at quarter-hourly intervals and was located at an artificially drained lowland site in the Spreewald region in north-east Germany. The results revealed that some of the monitoring stations were of value permanently while others were needed only temporally. The prevailing meteorological conditions, particularly the amount of precipitation, affected the degree of similarity between the water levels measured. The hydrological system tended to act more individually during periods of no or little rainfall. The optimal monitoring setup, its stability, and the monitoring effort necessary were influenced by the meteorological forcing. Altogether, the methodology presented can help achieve a monitoring network design that has a more even performance or covers the conditions of interest (e.g., floods or droughts) best.}, language = {en} } @article{FahleDietrich2014, author = {Fahle, Marcus and Dietrich, Ottfried}, title = {Estimation of evapotranspiration using diurnal groundwater level fluctuations: Comparison of different approaches with groundwater lysimeter data}, series = {Water resources research}, volume = {50}, journal = {Water resources research}, number = {1}, publisher = {American Geophysical Union}, address = {Washington}, issn = {0043-1397}, doi = {10.1002/2013WR014472}, pages = {273 -- 286}, year = {2014}, abstract = {In wetlands or riparian areas, water withdrawal by plants with access to groundwater or the capillary fringe often causes diurnal groundwater fluctuations. Various approaches use the characteristics of these fluctuations for estimation of daily groundwater evapotranspiration rates. The objective of this paper was to review the available methods, compare them with measured evapotranspiration and assess their recharge assumptions. For this purpose, we employed data of 85 rain-free days of a weighable groundwater lysimeter situated at a grassland site in the Spreewald wetland in north-east Germany. Measurements of hourly recharge and daily evapotranspiration rates were used to assess the different approaches. Our results showed that a maximum of 50\% of the day to day variance of the daily evapotranspiration rates could be explained by the approaches based on groundwater fluctuations. Simple and more complex methods performed similarly. For some of the approaches, there were indications that erroneous assumptions compensated each other (e.g., when overestimated recharge counteracted underestimated storage change). We found that the usage of longer time spans resulted in improved estimates of the daily recharge rates and that the estimates were further enhanced by including two night averages. When derived from fitting estimates of recharge or evapotranspiration with according measurements the specific yield, needed to convert changes in water level to water volumes, differed considerably among the methods (from 0.022 to 0.064). Thus, the specific yield can be seen as correction factor that compensates for inadequate process descriptions.}, language = {en} } @article{DietrichFahleSeyfarth2016, author = {Dietrich, Ottfried and Fahle, Marcus and Seyfarth, Manfred}, title = {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}, series = {Agricultural water management : an international journal}, volume = {163}, journal = {Agricultural water management : an international journal}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0378-3774}, doi = {10.1016/j.agwat.2015.09.005}, pages = {75 -- 89}, year = {2016}, 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. (C) 2015 Elsevier B.V. All rights reserved.}, language = {en} } @article{FahleDietrichLischeid2013, author = {Fahle, Marcus and Dietrich, Ottfried and Lischeid, Gunnar}, title = {A guideline for developing an initial hydrological monotoring network as a basis for water management in artificially drained wetlands}, series = {Irrigation and drainage}, volume = {62}, journal = {Irrigation and drainage}, number = {4}, publisher = {Wiley-Blackwell}, address = {Hoboken}, issn = {1531-0353}, doi = {10.1002/ird.1744}, pages = {524 -- 536}, year = {2013}, abstract = {Reliable hydrological monitoring is the basis for sound water management in drained wetlands. Since statistical methods cannot be employed for unobserved or sparsely monitored areas, the primary design (first set-up) may be arbitrary in most instances. The objective of this paper is therefore to provide a guideline for designing the initial hydrological monitoring network. A scheme is developed that handles different parts of monitoring and hydrometry in wetlands, focusing on the positioning of surface water and groundwater gauges. For placement of the former, control units are used which correspond to areas whose water levels can be regulated separately. The latter are arranged depending on hydrological response units, defined by combinations of soil type and land use, and the chosen surface water monitoring sites. A practical application of the approach is shown for an investigation area in the Spreewald region in north-east Germany. The presented scheme leaves a certain degree of freedom to its user, allowing the inclusion of expert knowledge or special concerns. Based on easily obtainable data, the developed hydrological network serves as a first step in the iterative procedure of monitoring network optimisation. Copyright (c) 2013 John Wiley \& Sons, Ltd.}, language = {en} }