Drainage Density Modeling using Precipitation in Different Climates of Kurdistan Province

Introduction

Drainage density is one of the most important geomorphologic indices of watersheds, which has often been used to express the degree of fluvial dissection, rainfall and infiltration capacity, flooding potential, landslide potential, topography evolution, and basin erosion. This index is greatly influenced by climate, vegetation, bedrock geology, time, and morphometric factors among which, the relationship between drainage density and climate is important, firstly in assessing the sensitivity of water resources and watershed hydrology to climate change; secondly, it determines how close to the truth the selection of this index is for climatic and hydrological studies; and thirdly, it reveales whether there is a significant difference in the amount of drainage density in different climates, so that it is necessary to consider its variability in the study of flooding, landslide and erosion potentials. The main objective of this study is to investigate the effects of climate (mean annual precipitation) on the watershed drainage density of Kurdistan Province, Iran through modeling this relationship, as well as drainage density variation with precipitation in different climates. This information can be utilized as key tools for watershed planning specifically in mitigating environmental crisis.



Materials and Methods

Study area

Kurdistan province is one of the provinces of western Iran, located between longitudes of 45º 31´ and 48º 16´ E and latitudes of 34º 44´ and 36º 30´ N. Its area is about 29137 Km2. The mean annual precipitation is 485 mm and the mean annual temperature is 9 ºC. This province is an important region since it is a huge source of fresh water for 5 large watersheds in Iran. The west of the province is covered by sparse oak forests while the east is mainly agriculture and rangelands. The majority of the province belongs to the Sanandaj-Sirjan geological zone and in terms of geomorphology, the entire Kurdistan province is covered by high mountainous areas and hilly regions.



Data Acquisition and Analysis

Based on long-term precipitation and temperature data of 8 synoptic weather stations of Kurdistan province, the climate was classified into four types including humid, semi-humid, Mediterranean, and semi-arid climates according to the De Martonne climatic classifications method by Surfer software. A number of 40, 87, 62 and 57 small watersheds (area less than 50 Km2) have been respectively selected from humid, semi-humid, Mediterranean, and semi-arid climates such that their total areas cover at least 10% - 15% of the entire area of each climate; at least, 40 to 100 watersheds exist in each climate for studying; and they have been distributed unformly in each climate. In order to determine the mean annual precipitation of each watershed, the iso-hyetal map of the Kurdistan province was prepared using the above-mentioned 8 weather stations in the ArcGIS environment. Drainage densities (Dd) of the 246 watersheds were calculated using Dd = ΣL/A equation. SPSS and Excel were used to analyze the data and to model the relationship between drainage density and mean annual precipitation in different climates of Kurdistan province.



Results and Discussion

Results showed that climate has a significant effect (significant level of 0.0001) on drainage density such that the mean of drainage density is significantly different in different climates of Kurdistan province. The maximum mean drainage density was 1.71 Km/Km2 occurred in Mediterranean climate while this climate has showed the minimum coefficient of variation for drainage density (19.9%) and maximum coefficient of variation for precipitation (21.9%). The minimum amount of drainage density was 1.12 Km/Km2 happened in humid climate while this climate showed a high coefficient of variation for drainage density (but not maximum value) and the minimum coefficient of variation for precipitation (10.2%). The modeling of drainage density with mean annual precipitation revealed a non-linear behavior of drainage density in different climates. The relationship of drainage density with mean annual precipitation in humid, semi-humid, Mediterranean, and semi-arid were modelled as exponential, linear, and sigmoid functions, respectively. We also came upon this result that the maximum drainage density has occurred in mean annual precipitation of 400 mm and in precipitations less or more than 400 mm, drainage density decreases. In precipitations less than 400 mm, drainage density increases with increase in precipitation, and in precipitations more than 400 mm, when precipitation increases, drainage density decreases. The 400 mm precipitation, therefore, is introduced as an Index Precipitation for studying morphometric factors specifically drainage density in Kurdistan watersheds and similar regions in future works. The findings of this study are in line with findings of Carlston (1963), Gregory and Gardiner (1975), Gregory (1976), Daniel (1981), Abrahams (1972), Abrahams and Ponczynski (1984), and Moglen et al. (1998) who had previously introduced 1250, 500, 500, 3000, 2000, 280, and 450 mm as Index Precipitation for studying drainage density in different climates at other parts of the world.



Conclusions

The drainage density of 246 small watersheds of Kurdistan province was investigated and its relationship with average annual precipitation was modeled and also the role of climate in the process of changes in drainage density was determined. The findings of this research showed that the climate has a significant effect on the drainage density of watersheds, therefore, in each climate, the relationship between the drainage density and the amount of precipitation has unique behavior and is different. In Mediterranean and semi-arid climates, changes in drainage density are under the control of precipitation, and in semi-humid and humid climates, changes in drainage density are under the control of vegetation cover, which should be investigated in future studies. A close relationship between drainage density and precipitation proved that this index can be used in the studies of climate change impacts on water resources and other natural hazards, however, it is necessary to consider its variability. Such studies can provide important information for official authorities to prepare better watershed management plans.