Effects of land-use changes on the hydrologic response of reclamation systems

Abstract

Our study investigates the possible effects of recent land-use changes on the frequency regime of floods for reclaimed lands. We modelled the runoff concentration behaviour of a reclaimed area of 76 km², located in the Po River plain near the city of Bologna (northern Italy), through the combined application of a semi-distributed rainfall-runoff model, which captures the key features of surface and sub-surface flows, and a hydrodynamic streamflow routing model. Three land use data from 1955, 1980 and 1992 were available. We implemented the rainfall-runoff model to the three land-use scenarios and analysed the hydrological–hydraulic behaviour of the study area for numerous rainfall events associated with different recurrence interval. The results of our study show a rather significant sensitivity of the flood frequency regime of the reclaimed land to land-use changes, and the sensitivity tends to increase as the recurrence interval of the rainfall event decreases.

Introduction

In the recent past, the question has often been raised about the possibility that the inundations that occurred in Europe in the last 40 years could be partially due to an increase of the flood risk, rather than being the result of natural climatic variability. This is in many cases alleged to be caused by man-induced land-use changes, which are mainly associated with increasing urbanisation and change of the agricultural practices. However, the different regional conditions usually prevent to arrive at universally valid statements about the land use impacts on the flood risk (Klöcking and Haberlandt, 2002).

Several recent studies addressed the quantification of the effect of land-use and land-cover change on the runoff dynamics under extreme hydrological conditions, focussing in particular on natural catchments (e.g., Hibbert, 1967, Lorup et al., 1998, Fohrer et al., 2001, Wooldridge et al., 2001, Brath et al., 2003a). Hibbert (1967) showed that there is clearly an increase in water yield due to reduction of forest cover, while he underlines the unpredictability of the response. After summarising a number of studies, Hollis (1975) concluded that while small frequent floods are increased many times by urbanisation, large rare floods are not significantly affected.

Different methodologies have been implemented in attempts to fill in the deficiency of knowledge of the subject, (see e.g., Kokkonen and Jakeman, 2002, Veldkamp and Fresco, 1996) and numerical rainfall-runoff models are being widely used with different approaches to assess the impact of land-use changes (see e.g., Hundecha and Bárdossy, 2004). Lorup et al., 1998, Schreider et al., 2002 calibrated lumped rainfall-runoff models for reference time-periods with known and stable land-use characteristics and then applied the calibrated models to subsequent time-periods, after major land-use changes. They implemented trend analysis to the bias between the modelled and the observed runoff to investigate changes in the catchment runoff that might arise from land-use changes. Fohrer et al. (2001) used a physically based model to investigate and predict the impact of land-use changes on the modelled flows. In order to achieve analogous objectives, Wooldridge et al. (2001) attempted to regionalize the parameters of a conceptual rainfall-runoff model for forested and non-forested catchments. Other investigations of the impact of land-use changes on hydrological processes are reported by Braud et al., 2001, Bultot et al., 1990, De Roo et al., 2001, Hundecha and Bárdossy, 2004, Robinson, 1998, Robinson et al., 2003.

The dramatic and steady expansion of urbanised areas is one of the most relevant land-use changes of the last five decades (see for instance Brath et al., 2003a, Brath et al., 2003b). Recent analyses, focussing on natural catchments, investigated the effects of such land-use changes on flood risk. The results of this kind of analysis are often puzzling and contradictory. Some of them point out that land-use changes may affect significantly the flooding potential of natural catchments (see e.g., Brath et al., 2003a), whereas some other claim that land-use changes have only a small effect on the hydrological response of natural catchments (see e.g., Langford, 1976). The fractions of urbanised areas in natural catchments may, in part, explain these divergent outcomes. In fact, the fraction of urbanised areas varies significantly from catchment to catchment and in several cases can be very limited (e.g., some percent).

One may expect to obtain more defined outcomes for reclaimed lands, as in these areas, otherwise inhospitable and infertile, man-induced modifications and human activities play a crucial role (e.g., regimentation and channelling of surface water, modification of soil permeability due to agricultural practices etc.).

The main objective of our study is to investigate the effects of historical land-use change on the flood risk of reclaimed lands. The study area is a reclaimed land of 76 km² located in the plain of the Po River near the city of Bologna (northern Italy). We implemented a semi-distributed rainfall-runoff model to different historical land-use scenarios from 1955 to 1992 and considered rainstorms associated with different exceedance probabilities, so as to quantify the impacts of the observed land-use dynamics on the flooding potential of the study area.