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Assessing the effects of land use change on the hydrologic regime
by RS and GIS - A case study in the Minab catchment, Hormozgan province, Iran
Jalal Barkhordari, Mahmud Damizadeh Agricultural and Natural Resources Research Center of Hormozgan Province, P.O. Box 79145-1577 Imam Khomaini Ave. Bandar-Abbas, Iran, Email: jbarkhordary@yahoo.com ABSTRACT This is illustrated by frequent occurrences of severe droughts and floods. The challenge is to determine whether these hydrological hazards and disasters are the result of climatologic variability or of man-induced changes. This study provides an alternative approach to assess the actual changes in hydrologic response of a watershed in an arid tropical region to land use transformations made in the past 25 years. The approach combines remotely sensed image data from satellites with in-situ hydrological observations from the Minab catchment (ca. 1.106 ha). Results of long-term analysis of historical time series on rainfall, land use and stream flow are integrated at the landscape level, to identify appropriate options for land and water management. In 1976, about 45 percent of the watershed area was covered by rangeland and natural forest. Due to continued overgrazing, rangeland cover decreased to 8 percent in 2002. Three main land use classes have replaced these fertile rangelands. These are: poor natural cover, agricultural area and residential area. The destruction of natural vegetation resulted in a decrease of the annual total water yield by 3.4 mm, with a decrease in the base flow during the low-flow period (May-November) and an increase in the storm runoff during the high-flow period (December to April). It can be concluded that climatic variability and land use change are the most important factors affecting the (changes in the) hydrologic regime of the Minab catchment. For a flood return period of more than 10 years, (high) rainfall intensity as a climatic factor is considered dominant. For a return period of less than 10 years in combination with a low flow period, land use change is the dominant factor determining the flow regime. An active management strategy aimed at the conservation and regeneration of the natural vegetation is recommended, in order to improve the distribution of water throughout the entire Minab catchment, during both dry and wet periods. Introduction Watershed analysis provides a framework for ecosystem management, which is currently the best option for conservation and management of natural resources. The water cycle regulates and reflects the natural variability of the physical processes which impact on ecosystems. With the ongoing development of land and water resources for agriculture and urban areas, hydrological regimes are changing to an increasing extent. This is illustrated on a global scale by more frequent occurrences of severe periods of drought and flood events. The challenge is to determine whether these hydrological effects are the result of (natural) climatologic variability or land use change by man. This study provides an alternative approach to ascertain the actual changes in hydrologic response of a watershed in an arid tropical region to land use transformations made in the past 25 years. The approach combines remotely sensed image data from satellites with in-situ hydrological observations from the Minab catchment (ca. 1.106 ha) in the South of Iran. Research Objectives
The general methodology is depicted in fig. 1. It consists of two major components. These are: (1) spatial land use analysis and (2) historic hydrological time series analysis.  Figure. 1 Flowchart Land use analysis Landsat MSS and TM satellite imagery have been used to:
Figure. 2 Land use maps of the Minab catchment, 1976, 1988 and 2002 Hydrological analysis Time series of hydro(-meteoro)logical data have been analysed to:
 Figure 4. Relation of runoff parameters, (moving average 5-years)  Figure 5. Flow duration curve of Minab River (1975-2000) Results and Conclusions In 1976, about 45 percent of the watershed area was covered by rangeland and natural forest (referred to as medium rangeland, good rangeland and forest). Due to continued overgrazing, rangeland cover decreased to 10 percent in 1988 and to 8 percent in 2002. During a period of ca. 25 years, three main land use classes at a large scale have replaced these fertile rangelands. These are characterized by: poor natural cover (e.g. poor rangeland, bare soil and rock out crop), agriculture area (e.g. irrigated agriculture, rainfed agriculture and orchard) and residential area. The destruction of the natural vegetation has resulted in:
It can be concluded that climatic variability and land use change are the most important factors affecting the (changes in the) hydrologic regime of the Minab catchment. For a return period of more than 10 years, (high) rainfall intensity is considered dominant as a climatic factor. It illustrates the impact of extremely high rainfall in the wet period. Other factors such as land use change are considered less important then. For a return period of less than 10 years and in combination with a low flow period, land use change is clearly the dominant factor determining the flow regime. Therefore, an active management strategy aimed at the conservation and regeneration of the natural vegetation is recommended, in order to improve the distribution of water throughout the entire Minab catchment, during both dry and wet periods. Reference
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