Allan Rodhe

Prediction uncertainty of conceptual rainfall–runoff models caused by problems in identifying model parameters and structure

Abstract The uncertainties arising from the problem of identifying a representative model structure and model parameters in a conceptual rainfall-runoff model were investigated. A conceptual model, the HBV model, was applied to the mountainous Brugga basin (39.9 km”) in the Black Forest, southwestern Germany. In a first step, a Monte Carlo procedure with randomly generated parameter sets was used for calibration. For a ten-year calibration period, different parameter sets resulted in an equally good correspondence between observed and simulated runoff. A few parameters were well defined (i.e. best parameter values were within small ranges), but for most parameters good simulations were found with values varying over wide ranges. In a second step, model variants with different numbers of elevation and landuse zones and various runoff generation conceptualizations were tested. In some cases, representation of more spatial variability gave better simulations in terms of discharge. However, good results could...

Validation of a distributed hydrological model against spatial observations

In connection with climate change studies a new hydrologic field has evolved - regional hydrological modelling or hydrologic macro modelling, which implies repeated application of a model everywhere within a region using a global set of parameters. The application of a physically based distributed hydrological model ECOMAG to river basins within the NOPEX southern region with this purpose in mind is presented. The model considers the main processes of the land surface hydrological cycle: infiltration, evapotranspiration, heat and water regime of the soil, snowmelt, formation of surface, subsurface and river runoff and groundwater. The spatial integration of small and meso-scale non-homogeneity of the land surface is a central issue both for the definition of fundamental units of the model structure and for determination of representative values for model validation. ECOMAG is based on a uniform hydrological (or landscape) unit representation of the river basin, which reflects topography, soil, vegetation and land use. As a first step the model was calibrated using standard meteorological and hydrological data for 7 years from regular observation networks for three basins. An additional adjustment of the soil parameters was performed using soil moisture and groundwater level data from five small experimental basins. This step was followed by validation of the model against runoff for 14 years from six other drainage basins, and synoptic runoff and evapotranspiration measurements performed during two concentrated field efforts (CFEs) of the NOPEX project in 1994 and 1995. The results are promising and indicate directions for further research.

Transit Times for Water in a Small Till Catchment from a Step Shift in the Oxygen 18 Content of the Water Input

Transit times for water in a small till catchment from a step shift in the oxygen-18 content of the water input

Wetland occurrence in relation to topography : a test of topographic indices as moisture indicators

Information about the spatial distributions of soil moisture or groundwater levels is needed for aggregation of soil- vegetation-atmosphere-transfer (SVAT) models. The possibility of predicting wetness distributions in catchments from topographic data was investigated using topographic indices, notably the TOPMODEL index. The indices were calculated from commercially available gridded data (50 50 m 2 ) over two areas with contrasting topography: a catchment (Nasten) in the low-relief NOPEX region in southern Sweden and a group of catchments in a hilly area (Kassjoan) in central Sweden. The occurrence of mires, assumed to represent the extreme wetness end of the wetness spectrum, was used as field data. The frequency distributions of topographic indices for mire and non-mires were clearly different in Kassjoan, although there was a large overlapping, whereas the two distributions were very similar in Nasten. Prediction of mires from topographic indices was meaningful only in Kassjoan. Although it gave poor results in terms of fractions of successfully predicted mire cells out of the observed mire cells, the general spatial patterns of mires were fairly well simulated. One important reason for the failure of the indices to predict mires in Nasten, and probably also to predict other wetness classes, is that the spatial resolution in the index calculation was coarser than typical length scales of the topographic features in this catchment, being only a few tenths of meters. The importance of geologic conditions in modifying the topographic control over the wetness is exemplified from the obtained relationship between topographic indices and mire occurrence. # 1999 Published by Elsevier Science B.V. All rights reserved.

Hydrograph separation using continuous open-system isotope mixing.

We propose a new hydrograph separation method for runoff source modeling based on continuous open system isotope mixing using a variable source area and three isotopic reservoirs. The three reservo ...

Water transit times and flow paths from two line injections of 3H and 36Cl in a microcatchment at Gårdsjön, Sweden

To investigate groundwater transit times and flow paths in shallow till soil, within an acidification study at Gardsjon, Sweden, two line injections of Cl-36 and H-3 Were made in groundwater during ...

Transit times of water in soil lysimeters from modeling of oxygen-18

A proper description of water pathways and transit times is important in the simulation of groundwater acidification using hydrochemical models. A simple water balance model, describing water flow and transit times in different soil layers, was developed and tested by the use of the stable isotope 18O as a natural tracer in soil lysimeters. Drainage was collected from lysimeters of three depths: 15, 40, and 80 cm, from two sites in the Stubbetorp research basin in south-eastern Sweden. The content of 18O in the precipitation and in the drainage from the lysimeters was measured during 2 yr. 18O was regarded as an ideal tracer, and its concentration in the drainage was modeled using the concentration in the precipitation as input. The percolation from each soil layer was assumed to depend on the inflow and the soil moisture storage in the layer. The most important model parameter, the field capacity, was derived from field information. Sensitivity analysis showed that the model was rather insensitive to other parameter values. Although simple, the model gave good results, both for the flow of water and 18O. The best results were obtained, when ideal mixing in the upper horizons of the soil was combined with piston flow at greater depths. Preferential flow was not found to be of great importance, nor was immobile water. Particle flow velocities and transit times in the soil lysimeters were simulated. The average particle flow velocities were about 0.6 cm d−1. The use of a dynamic model made it possible to simulate the temporal variations in transit times for water in the soil lysimeters. The mean transit times for the 80 cm lysimeter ranged from about 3 to 6 mo with an average value of 4 mo.

Kinematic wave approximations to hillslope hydrological processes in tills.

This work has been carried out within the framework of NOPEX - a NOrthern hemisphere climate Processes land surface EXperiment. Its purpose is to describe the spatial variability of groundwater lev ...

Importance of hydrology in the reversal of acidification in till soils, Gårdsjön, Sweden

In order to determine, experimentally, the rate at which surface water quality recovers after acid deposition is reduced, a roof was built over a small catchment near Lake Gardsjon, southwest Sweden. By irrigating the catchment with chemically adjusted water, the input of S and N is reduced by at least 90%. Previous hydrological research on catchments similar to that used in the roof experiment suggested that storm runoff from the catchment is concentrated in surface-near soil layers, temporarily saturated when the water table has risen in response to rainfall. If so, it is suggested that the improvements in water quality using the roof catchment will appear first in storm runoff, because the spate-specific flow pathways will be flushed by the large throughput of the non-acidic irrigation water. The hydrological basis of this suggestion was investigated using hydrometric observations and tracer studies. Laboratory analysis of ~100 soil samples gave mean values of the saturated hydraulic conductivity of 1 × 10−4 m/s in the uppermost 20 cm of the soil layer, decreasing to 6 × 10−6 m/s in the 20 cm closest to bedrock. The variation in hydraulic conductivity roughly agreed with the one inferred from the observed relation between groundwater level and catchment runoff. A preliminary tracer experiment with injected 3H and 36Cl, conducted during a very wet period, showed particle velocities of >1 m/h in the soil. This was significantly higher than velocities estimated from hydrometric observations, being ~0.1 m/h in the A horizon and ~0.01 m/h in the less conductive B and C horizons. The greater particle velocity indicated by the tracer experiment suggested that much of the flow was concentrated in a small fraction of the pore space. The replacement of the pre-irrigation water in the catchment by irrigation water could be investigated by using 18O, because the 18O content of the irrigated water differed from that of the groundwater and soil water before the irrigation. The 18O data showed that after six months of irrigation, about one fourth of the baseflow was new water.

Urban storm water transport and wash-off of cæsium-137 after the Chernobyl accident

Radiation protection planning in urban areas after a radioactive fallout requires knowledge of decontamination caused by storm water transport. This report elucidates the transport of 137Cs from a roof and three storm sewers in Uppsala during the first rainfall after those, on 29–30 April 1986, causing the Chernobyl contamination. Runoff and concentration Of 137Cs in storm water were determined with an accuracy of 10 to 15%. The origin of storm water was determined from its content of 18O. Surface contamination was measured on 3 July 1986. Total fallout was 25 kBq m−2. During the rainfall on 11 May, 4 kBq m−2 were transported from the roof and approximately 1 kBq m−2 from the sewered areas. From 30 April to 4 July there was a decontamination of 13 to 20 kBq m−2. Measurements from another project showed that the fallout rain events washed off 10 to 16 kBq m−2. The wash-off by the small rainfall on 11 May constituted one third of the remaining decontamination occurring up until 4 July. The transport of 137Cs during the 11 May event increased when the runoff increased, but was less efficient as the event proceeded. The relations between 137Cs concentrations and runoff implied that the wash-off of 137Cs in Uppsala was totally dominated by that bound to particles.