Impacts of land-use and land-cover changes on flow regimes of the Usangu wetland and the Great Ruaha River, Tanzania

Abstract

A study was conducted to investigate the hydrological impacts of land-use and land-cover changes on flow regimes of the Great Ruaha River. Remote sensing and GIS techniques were used to inventory temporal changes of land-use and land-cover changes in the watershed. Hydrological data were analyzed to reveal the alterations and trends for three time periods; pre-1974, 1974–1975, and post-1985. It was revealed that there was a steady increase in cultivated area, from 121.2 km² to 874.3 km² between 1973 and 2000 while the woodland area decreased significantly over years. The minimum dry season area of the wetland declined significantly, with major changes occurring between 1984 and 2000. River flows were found to be highly variable within and between the years, and sensitive to land-use and land-cover changes. There was a slight shift of the time peak flows, from April to early February during the post-1985 period. The Q₅₀ flow progressively declined from 19.23 m³ s⁻¹ in the pre-1974 to 16.51 m³ s⁻¹ and 9.04 m³ s⁻¹ for 1974–1975 and post-1985 period. The Q₉₅ (low flow) declined from 2.84 m³ s⁻¹ to 0.11 m³ s⁻¹ and 0.0 m³ s⁻¹ for pre-1974, 1974–1975 and post-1985 period. The study concludes that the modification of the land use and cover has resulted in changes in temporal distribution of runoff. The study highlights the importance of considering effects of land-use and land-cover changes on ecosystems and water resources for an informed decision on proper catchment planning and management.

Introduction

Understanding the influence of land-use and land-cover change on river flow regimes is important for sustainable catchment management. Worldwide it has become evident that river ecosystems have changed as a result of river regulation modifying the flow regime (Postel and Richter, 2003, Brown and King, 2003). Also, the increased competition for water and alterations in land use in the upstream of many rivers, are argued to have contributed to change in hydrological regimes of many rivers and wetlands.

Against this background, this paper presents a study undertaken to investigate the influence of land-use and land-cover changes on the hydrological regimes of the Great Ruaha River (GRR) downstream of the Usangu Plains in Tanzania (Fig. 1). The Great Ruaha River is a major tributary of the Rufiji River. In terms of the national economy it is one of the country’s most significant waterways, with more than 50% of the country’s hydropower capacity and significant agricultural production (Kadigi et al., 2004). Furthermore, it is the main source of water during the dry season, and so is vital for the ecology of the Ruaha National Park. Since 1992/93, the previously perennial river ceased flowing every dry season and the early part of each wet season (i.e., September–January) downstream of the Eastern (Ihefu) wetland. Various studies have been conducted in the area trying to investigate the possible causes for the changes and options for mitigation of the prevailing effect on the Great Ruaha River environment especially the Ruaha National Park (i.e., Baur et al., 2000, Kikula et al., 1996, Yawson, 2003, Machibya, 2003).

Although previous studies have been carried out in the Usangu Plains (Moirana and Nahonyo, 1996, SMUWC, 2001a, Mtahiko et al., 2006), none of these explicitly investigated the influence of land-use change on river flow regimes. The current study sought to improve understanding of the influence of land use and cover changes on hydrological regimes of the GRR. The study acknowledges the fact that formulation of appropriate strategies to manage water resource depends on a sound understanding of the causes of the problems and the impacts it has on the downstream part.