A distributed modelling system for simulation of monthly runoff and nitrogen sources, loads and sinks for ungauged catchments in Denmark

Abstract

We validated an existing physically based 3D MIKE SHE groundwater resource model (DK-model) at 175 Danish gauging stations covering different catchment sizes in order to calculate monthly water runoff in the 50% ungauged part of Denmark. Model performance was in most cases good (61% of gauging stations had a Nash-Sutcliffe (NS) coefficient >0.60) but nevertheless showed a large seasonal and georegion specific bias. Therefore, bias correction factors had to be developed before applying the DK-model simulations of runoff in the ungauged areas. Simulated monthly runoff from ungauged areas and the measured monthly runoff from 178 gauging stations were distributed to 2663 smaller Hydrological Units (ca. 15 km(2)) and linked with a new empirical model for flow-weighted monthly total nitrogen (TN) concentrations (R(2) = 0.43; P < 0.0001) developed based on 20 years of observations (1990-2009) in 83 small catchments for calculation of monthly gross diffuse TN-loads from HU's. Nitrogen retention was calculated in streams, lakes and wetlands utilising both lake specific models and rate coefficients to calculate N retention in surface water bodies. The whole model complex was linked in the DK-QN concept for simulation of monthly TN losses from point sources and diffuse sources, TN retention and resulting loadings to Danish coastal waters. The DK-QN model was validated in 118 gauged catchments and the model simulations had for >25% of the observations of monthly discharge weighted TN concentrations a NS larger than 0.26. Catchment specific monthly TN-loadings were modelled with a higher performance as 50% of the catchments had a NS greater than 0.75. The model concept allows calculation of N retention in streams, lakes and wetlands and the average annual model calculated N retention amounted to 21% of the modelled gross riverine TN loadings. The average annual gross TN loading to surface freshwater in Denmark derived from diffuse sources amounted to 97 000 tonnes N (91% of gross TN loadings) which is 54% of the total estimated N-leaching from the root zone on the Danish land area (212 000 tonnes N) during the period 1990-2009.

Threshold values and management options for nutrients in a catchment of a temperate estuary with poor ecological status

“…most recently the requirements described in the EU's Monitoring Directive (EU, 2009). Selected data from the monitoring programs are shown in Tables 1 and 2 Kronvang et al, 2009b), an empirical monthly flow-weighted N concentration model from diffuse sources (DK-QN, Windolf et al, 2011), and a physically distributed integrated hydrological model (DK-model, Henriksen et al, 2003).…”

“…The model simulations of nitrogen leaching and the modeled gross and net nutrient emissions are believed to be of relatively high precision as the models applied are empirical models developed based on the national monitoring data from agricultural fields in agricultural catchments (Grant et al, 2007) and stream monitoring data from 80 catchments (Windolf et al, 2011). This conclusion is corroborated by the good fit to the measured stream concentrations in the gauged sub-catchments (Table 2).…”

“…1). Land-based monthly nutrient loadings and freshwater discharge from the entire catchment to the Horsens estuary for the period 1984 to 2009 have been estimated utilizing data from the two gauged stations, and adding modeled monthly freshwater discharge and nutrient loadings from the ungauged part of the catchment by using the DK-QN model complex according to Windolf et al (2011) (Fig. 2).…”

“…The model was developed based on nitrogen data for 83 small agricultural catchments without lakes or wetlands and data for the period 1990 to 2009 using an approach described by Kronvang et al (1995), Andersen et al (2005), and Windolf et al (2011). The retention of TN in streams, lakes and wetlands was calculated utilizing different models and expert judgments as described in Windolf et al (1996Windolf et al ( , 2011 and Kronvang et al (2005). The modeling complex allowed a model estimation of gross and net stream flow-weighted concentrations taking into consideration the nutrient retention in the 5 larger lakes situated in the catchment, of which the 2 largest are situated downstream the two monitoring stations just before river water enters the estuary (Fig.…”

Nitrogen and phosphorus transformations and balance in a pond-ditch circulation system for rural polluted water treatment

“…Whenever water resources management of a large river basin is urgently required, the hydrological processes of small catchments, which are basic hydrologic units, are needed. Because of the lack of long-term runoff data for the small basins, the physically based distributed models is the only available solution to simulate the processes, since a model based on field-measured data is generally better able to represent natural watershed properties and can efficiently improve the simulation accuracy of these ungauged regions (Windolf et al, 2011).…”