عنوان مقاله [English]
Gullies are an important part of the soil erosion process and their occurrence and development may cause serious problems to a region s economy. They erode agricultural lands and act as potentially important sediment sources for rivers and reservoirs. Gully processes have been studied from a range of viewpoints (e.g. hydraulics, agricultural and forest engineering, hydrology and geomorphology) but despite a wealth of information collected, particularly in the last 20-30 years, many problems of morphology remain poorly known. Attempts to define gullies and their typology reflecta discouraging diversity of opinions. As far as gully spreading is concerned some authors believe that because of the scarcity of data there is an insufficient knowledge of gully development. Recent studies indicate that (1) gully erosion represents an important sediment source in a range of environments and (2) gullies are effective links for transferring runoff and sediment from uplands to valley bottoms and permanent channels where they aggravate off site effects of water erosion. In other words, once gullies develop, they increase the connectivity in the landscape. Many cases of damage (sediment and chemical) to watercourses and properties by runoff from agricultural land relate to (ephemeral) gullying. Consequently, there is a need for monitoring, experimental and modelling studies of gully erosion as a basis for predicting the effects of environmental change (climatic and land use changes) on gully erosion rates. In this respect, various research questions can be identified.
The study area is located in North of Darab province and south of Fasa province, southwest Iran. Surface soil samples (0-20 cm and 20-60 cm) were sampled from gully of the study area, air-dried and sieved (<2 mm) for laboratory analyses. Then some physical and chemical properties were determined. Particle size distribution was determined by the method of Rowell (1994). Calcium carbonate equivalent was determined by acid neutralization (Salinity Laboratory Staff, 1954). Organic carbon was determined by the method of Nelson and Sommers (1982). The soil pH was determined in saturated paste (Salinity Laboratory Staff, 1954). Electrical conductivity was determined in the saturated extract (Salinity Laboratory Staff, 1954). Cation exchange capacity (CEC) was determined using sodium acetate at a pH of 8.2 (Chapman, 1965). Available K was determined by the method of Pratt (1965). Then K was measured on all filtrated extracts by Corning 405 flame photometer. Plant available soil phosphate was measured by the method of Olsen et al. (1954). Also In order to understand the factors influencing gullying a detailed inventory of the gullies (number, density, size, type, location in the drainage basin etc.) was completed. Topographical maps and aerial photographs have been used to provide this information as in India (Haigh, 1984), Tadjikistan (Ahmadov, 1977), Russia, Bielorussia, Ukraine(Kosov and Kostantinova, 1972), Lesotho (Nordstrom, 1988), Brasilia (De Oliveira,1990), Israel (Seginer, 1966), and Romania (Motoc et al., 1979). The inventory of gullies in this study used topographical maps at a scale of 1 : 25,000. For each case the following variables were taken into consideration: number of gullies, length, maximum width and maximum depth, gully orientation, depth of head cat and so on. Finally for determination of relationship between gully characteristics and soil characteristics used regression.
The general purpose of multiple regressions is to learn more about the relationship between several independent or predictor variables and a dependent or criterion variable. The general form of the regression equations is according to Eq. 1:
Y=A0 + A1X1 +A2X2 + … +bnXn (1)
Where Y is the dependent variable, A0 is the intercept, A1. . .bn are regression coefficients, and X1– Xn are independent variables referring to basic soil properties.
Results and discussion
The results show that there are high correlation between the volume of gully and depth of gully. Also the correlation between volume of gully and soil properties (sodium, potassium, magnesium, chlorine, zinc, copper, iron, bicarbonate and organic carbon) showed that by reducing the amount of organic matter (OC) and bicarbonate (CC) increase volume of gully. So that the OC with a correlation coefficient CC -0.987** and -0.987** are in reverse correlation with the Volume of gully.
In this study it was found that by reducing soil nutrient increases the size of the gully in the study area. It was also found that increasing human activities in areas where soils are susceptible to erosion, gully expansion will increase.