Hunting for Information in Streamflow Signatures to Improve Modelled Drainage

About half of the Danish agricultural land is drained artificially. Those drains, mostly in the form of tile drains, have a significant effect on the hydrological cycle. Consequently, the drainage system must also be represented in hydrological models that are used to simulate, for example, the transport and retention of chemicals. However, representation of drainage in large-scale hydrological models is challenging due to scale issues, lacking data on the distribution of drain infrastructure, and lacking drain flow observations. This calls for more indirect methods to inform such models. Here, we investigate the hypothesis that drain flow leaves a signal in streamflow signatures, as it represents a distinct streamflow generation process. Streamflow signatures are indices characterizing hydrological behaviour based on the hydrograph. Using machine learning regressors, we show that there is a correlation between signatures of simulated streamflow and simulated drain fraction. Based on these insights, signatures relevant to drain flow are incorporated in hydrological model calibration. A distributed coupled groundwater–surface water model of the Norsminde catchment, Denmark (145 km2) is set up. Calibration scenarios are defined with different objective functions; either using conventional stream flow metrics only, or a combination with hydrological signatures. We then evaluate the results from the different scenarios in terms of how well the models reproduce observed drain flow and spatial drainage patterns. Overall, the simulation of drain in the models is satisfactory. However, it remains challenging to find a direct link between signatures and an improvement in representation of drainage. This is likely attributable to model structural issues and lacking flexibility in model parameterization.

Hydrogeological, topographical and drain factors controlling water table depth variation and potential nitrate reduction in subsurface drained clayey till area

<p>Nutrient losses in agricultural areas have detrimental effects not only on the surface water quality but are also unfavorable for sustainable agriculture practices. In Denmark, there are currently nitrate regulations applied for ID15 catchments (15 square km scale), nevertheless, it is crucial to know how much nitrogen is retained in the root zone, saturated zone, riparian zone as N-retention varies widely within ID15 catchments. Currently, N-retention mapping does not incorporate N-retention in the root zone on ID15 scale. N-retention in the root zone of subsurface drained clayey areas is potentially influenced by the variation in water table depth. Therefore, we will evaluate the effect of shallow hydrogeology, topography and drain parameters on local (sub-field scale) water table depth variation using a case study in eastern Jutland, Denmark.</p><p>The aim of the study was to assess which hydrogeological variables, drain parameters and topographical variables control water table depth variation in the root zone. This analysis was aided by a groundwater flow model code (MODFLOW). For the following purpose, hydrological data (drain flow at the outlet and depth to the water table in piezometers) and geophysical data (subsurface electrical conductivity) were collected. The geophysical data was collected by two ground-based electromagnetic systems (DUALEM and tTEM). The electrical conductivities were directly translated into two zones of homogeneous hydraulic conductivities based on a threshold value. Hydraulic properties were varied for each zone. Areas with no geophysical data were simulated using Direct Sampling, a Multi Points Statistics method. We generated several flow models, which had a varying spatial distribution of hydraulic zones and varying hydraulic properties (input factors). Moreover, boundary conditions (lateral fluxes), topographical smoothing and drain parameters (drain conductance and drain depths) were some of the other input factors we considered in this work. Model boundary conditions data were obtained from the national hydrological model. The variation in input factors was related to variation in simulated water table depths and drain flow at the outlet using a one-at-a-time sensitivity analysis.</p><p>Drain flow fraction, depth to the water table and drain discharge are analyzed as the quantity of interest for both wet and dry periods. Drain fraction is calculated as the ratio of the area contributing to the drainage to the area contributing to the recharge within the same area. The results will discover crucial controlling components of water table depth with which variations in N-retention can be estimated between different fields. The emphasis is to discover the connection between hydrogeological, topographical, and drain variables, and water table depth. We will examine potential implications for evaluating drain fraction and potential nitrate reduction.</p>

Complex of structures for utilization of saline drain flow

The article describes a complex of structures for utilization of saline drain flow on account to pre-accumulation and intensified evaporation of drainage water, concentration and use of concentrated brines and sedimentary salts. Intensification of evaporation is provided by increasing the water temperature and the area of the evaporating surface as a result of aeration with preheated air.

Including hydrologic signatures in the calibration of a groundwater-surface water model to improve representation of artificial drain

<p>About half of the Danish agricultural land is artificially drained to make land arable and increase crop yield. Those artificial drains, mostly in the form on tile drains, have a significant effect on the groundwater flow patterns and the whole water cycle. Consequently, the drainage system must also be represented in hydrological models that are used to understand and simulate, for example, recharge patterns, groundwater flow paths, or the transport and retention of nutrients. However, representation of drain in regional- and large-scale hydrological models is challenging due to i) issues with scale, ii) a lack of data on the distribution of the drain network, and iii) a lack of direct observations of drain flow. This calls for more indirect methods to inform such models.</p><p>We assume that drain flow leaves a signal in certain hydrograph signatures, as it impacts the generation of streamflow. Based on a dataset of observed discharge covering all of Denmark, and simulation results from regional-scale hydrological models, we use machine learning regressors to shed light on possible correlations between hydrograph signatures and artificial drainage. Building up on this step, we run a series of calibration exercises on a hydrological model of the agriculturally dominated Norsminde catchment, Denmark (~100 km<sup>2</sup>). The model is set up in the DHI MIKE SHE software, as distributed coupled groundwater-surface water models with a grid size of 100 m. The different calibration exercises differed in the objective functions used: either we only use conventional stream flow metrics (KGE), or also include hydrograph signatures that showed sensitive towards drain flow in our regression analysis. We then evaluate the results from the different calibration exercises, in terms of how well the model reproduces directly observed drain flow, and spatial drainage patterns.</p><p>Despite including hydrologic signatures in the calibration process, the representation of drain flow in large-scale models remains challenging. Eventually, the insight gained from this and similar studies will be incorporated in the National Water Resources Model for Denmark, to help improving national targeted regulation of nitrate application through fertilizers.</p>

Development and Sensitivity Analysis of an Online Tool for Evaluating Drainage Water Recycling Decisions

HighlightsA modeling framework for drainage water recycling (DWR) was developed to estimate irrigation and water quality benefits.Global sensitivity analysis was used to identify most and least influential input parameters affecting model outputs.Parameters controlling total available water had the most influence on applied irrigation and captured tile drain flow.The modeling framework and sensitivity results were used to develop an open-source, online tool for evaluating DWR.Abstract. The U.S. Midwest is experiencing growth in both irrigation and subsurface (tile) drainage. Capturing, storing, and reusing tile drain water, a practice called drainage water recycling (DWR), represents a strategy for supporting supplemental irrigation while also reducing nutrient loads in tile-drained landscapes. This article describes the development and testing of an open-source online tool, Evaluating Drainage Water Recycling Decisions (EDWRD), which integrates soil and reservoir water balances for a tile-drained field and estimates potential benefits of DWR systems across multiple reservoir sizes. Irrigation benefits are quantified by applied irrigation and its relation to the irrigation demand, while water quality benefits are quantified by the amount and percentage of tile drain flow captured by the reservoir. Global sensitivity analysis identified input parameters affecting total available water as the most influential factors in estimating outputs. Initial and mid-season crop coefficients, irrigation management, and reservoir seepage rates were also influential. Curve number, fraction of wetted surface during irrigation, crop coefficients for the end of crop growth and frozen soil conditions, and the non-growing season residue amount were identified as low-sensitivity parameters. Results from the sensitivity analysis were used to prioritize and simplify user interaction with the tool. EDWRD represents the first open-source tool capable of evaluating DWR systems and can be used by multiple user groups to estimate the potential irrigation and water quality benefits of this innovative practice. Keywords: Drainage water recycling, Dual crop coefficient, Open-source model, Sensitivity analysis, Subsurface drainage, Supplemental irrigation.

EFFECTS OF CONTROLLED DRAINAGE ON SOIL WATER REGIME AND QUALITY IN LITHUANIA

Lithuania remains one of the most extensively drained of the Baltic and Nordiccountries. The overall drained area (ditches plus tile drains) totalled 87% of theagricultural land area. Many nutrients from soil are leached through drainageresulting in polluting streams (drain flow receivers) water. Drain flow is treated asa major determinant of water quality. Therefore, the reduction of nutrients enteringthe drains is very important. Controlled drainage conception, when the outflowheight is increased at the mouth, helps reduce drainage runoff and partially purifywater. The aim of the research was to establish controlled drainage influence on thesoil moisture regime, nitrogen and phosphorus leaching. Investigations werecarried out in sandy loam and loam soils in the Middle Lithuanian Lowland. Basedon studies, several tendencies were observed: when drainage outflow began, theamount of soil moisture in subsoil (50-80 cm layer of the soil) of controlleddrainage plot was higher than in the conventional drainage plot, and highermoisture supplies stayed for a longer period of time. Controlled drainage had nodirect impact on phosphorus and nitrogen concentrations but they were influencedby the leaching quantities of plant usable nutrients. The reason that in many caseslower nitrate nitrogen (54% of all measurements) and phosphorus concentrations(77% of all measurements) were found in the conventional system rather than inthe controlled drainage might be connected to the fact that the latter area containedpredominantly lighter textured soils (sandy loam) making it easier to wash awaythe nutrients unused by plant.

Determination of Tile Drain Discharge under Variable Hydraulic Conditions

The aim of this study was an investigation of tile drain flow velocity under variable hydraulic conditions and of tile drain discharge using an ultrasonic flow meter. There is an essential variance between velocity values measured by an ultrasonic flow meter and reference values determined by a laboratory method. Differences result from specific measurement conditions, which appear in drainage pipe systems. The values of velocity measured by ultrasonic flow meters were higher than the reference values for three examined flow phases: Free, transient, and pressured flow. The discharge from a tile drain working on a partially filled up pipe in no-pressure conditions should be calculated by an adapted equation based on the California pipe method, whereas the discharge from a completely filled up drain pipe working over pressure should be calculated as a ratio between 0.428 times the measured velocity and the pipe cross-section area.