Simulation of the impacts of land use/cover and climatic changes on the runoff characteristics at the mesoscale
Introduction
A mesoscale river basin1 is an open, highly complex dynamic system with poorly controlled boundary conditions, whose complexity stems from the fact that it comprises a large number of agents (e.g. people, firms, and government) and a number of tightly coupled subsystems interacting in both space and time at various scales Weidlich and Haag, 1983a, Blöschl and Sivapalan, 1995, Allen and Strathern, 2003, Allen, 2004. These subsystems have very different characteristics but most of them – if not all – exhibit intrinsic nonlinear relationships, delayed responses, feedback loops Wu and Marceau, 2002, Porporato and Ridolfi, 2003 and, in some cases, stochastic behaviours. As a result, the macrostates that emerge from such a system may range from stochastic to chaotic – in the long-term – as was shown by Jayawardena and Lai, 1994, Sivakumar, 2000, and Sivakumar et al. (2002) among others. Moreover, it is recognised that there is a considerable uncertainty in our knowledge of the component processes and the representation of that knowledge when one builds predictive models for such a system Refsgaard, 1997, Beven, 2002, Young and Parkinson, 2002.
Despite the intricacies of this system, remarkable progress in analysing it has been achieved during the past decades. The main approach employed consists of subdividing it into sub-components which are, in turn, modelled as uncoupled subsystems having no endogenous links to any other one. For example, land use models and river basin econometric models were not coupled with hydrological ones [e.g. Berry et al., 1996, Guise and Flinn, 1973, respectively] and, conversely, precipitation-runoff models neither consider models dealing with the land use dynamics nor the economic development of the region [e.g. Leavesley et al., 1983 among others]. The main shortcomings of the lack of integration are: (1) the loss of both the synergy and the feedback effects among subsystems, and (2) the impossibility of estimating the degree to which a variable in a given subsystem may be changed due to the increase or decrease of another one belonging to an exogenous subsystem.
The former is evident when one tries to assess the most likely impacts that land use/cover and climatic changes would induce on the runoff characteristics (e.g. total discharge in winter or frequency of high flow in summer) at the mesoscale (Samaniego, 2003). In order to deal with this problem properly, a holistic formal system (Casti, 1984)– or model – that mimics a number of links (most of them nonlinear) among the relevant processes (e.g. macroclimatic conditions, soil type distribution, hydrological regime) and the driving forces behind land use/cover change should be formulated. There are several studies reported in the literature dealing with the integration of land use/cover change and hydrological models, for instance those of Bronstert et al. (2002) and Niehoff et al. (2002). In those studies, however, the treatment of the land use/cover change model did not follow a stochastic framework.
This paper presents an attempt to develop a simple but robust integrated model to address this problem, as well as the main assumptions therein, and to propose suggestions for future improvements. The proposed approach is illustrated by a case study carried out in a river basin in Germany.
Section snippets
Method
Building an integrated model for an environmental system requires trade-offs between the level of complexity and the degree of coupling among the various subsystems. The former is determined by the number of state variables and parameters as well as their spatio-temporal scales, whereas the latter is given by the number and complexity of the coupling functions and/or interface variables (Walter and Liossatos, 1979). The model, however, should (1) provide a sufficiently good resemblance of the
The study area
The proposed simulation model was applied in the catchment located upstream of the gauging station Denkendorf-Sgewerk in the river Körsch, Germany (see Fig. 2). Its area is 126.3 km2 and because of its vicinity to Stuttgart it has endured a rapid land use/cover change in the past four decades – mainly from agricultural areas to settlement – reaching a gross density of about 720 in./km2 by the end of 2001 (Samaniego, 2003).
Calibration and validation of the LUCC model
Considering the data availability for the study area, three ()
Results and discussion
The behaviour of the system, roughly represented by the model , was assessed by 2500 realisations under each set of development scenario conditions. In all these simulations, 1994 was used as the reference year. The simulation period was set from 1994 to 2025 with yearly time steps. As a summary of these simulations, the average growth rate in percent per decade for each simulated variable as well as the probability that the long-term mean of a given variable will be exceeded were estimated,
Conclusions
Considering the basic objective proposed at the beginning of this paper, it is possible to draw the following conclusions:
- (1)
Coupling parsimonious hydrological models of several runoff characteristics at the mesoscale with a simple stochastic land use/cover change model has proved to be feasible and enlightening. The approach followed here not only allows one to relate a number of variables otherwise exogenous to the system but also to produce results that highlight the uncertainty of each
Acknowledgement
The authors appreciated the helpful comments from two anonymous reviewers.
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