Class 12 Factors affecting watershed management.pptx

Geography

1. Delivered by Mr. Anil Swami Asst. Professor 1 Rainfed Agriculture & Watershed Management Mr. Anil Swami Factors affecting watershed management

2. Objective:- Tell the soil and climatic conditions prevalent in rainfed areas. Interpret various water harvesting techniques and their efficient utilization. Apply contingent crop planning for aberrant weather conditions. Examine the seasonal rainfall and different types of watershed and its components. Select soil and water conservation techniques to avoid their losses. 2

3. The factors affecting watershed management are divided into five groups: 1. Watershed characteristics 2. Climatic characteristics 3. Land use patterns 4. Social status of inability 5. Organization

4. 1. Watershed Characteristics Watershed characteristics influencing watershed management include size, shape, topography, slope, soils, vegetative cover and drainage density. Size Size of watershed determines the quantity of rainfall received, retained and surface runoff. A small watershed is pronounced by overland flow which is the main contributor to peak flow. The longer the watershed, the higher is the time of concentration while broader the watershed the smaller is the time of concentration. A large watershed has no overland flow, but channel flow is significant. Large watersheds are also affected by basin storage.

5. Shape Shape contributes to the speed with which the runoff reaches the river. A long catchment area will take longer to drain than a circular catchment. Basin shape is not usually used directly in hydrologic design methods. A circular shaped watershed would results in runoff from various parts of the watershed reaching the outlet at the same time. An elliptical watershed having the outlet at one end of the major axis (having the same area as the circular watershed) would cause the runoff to spread out over time, thus producing a smaller flood peak than that of circular watershed.

6. Topography Topographic configuration like slope, length, degree and uniformity of slope affect both disposal of water and soil loss. Time of concentration and infiltration of water are a function of degree and length of slope. Topography determines the speed with which the runoff will reach a river. If the land slope increases four times, the velocity of water flowing down the slope is doubled. When velocity is doubled, the erosive capacity increased four times. Clearly, rain that falls on steep mountainous areas will reach the primary river in the watershed faster than flat/slopping areas.

7. Slope Watershed slope affects the momentum of runoff. Both watershed and channel slope may be of interest. Watershed slope reflects the rate of change of elevation with respect to distance along the principal flow path. It is usually calculated as the elevation difference between the end points of the main flow path divided by the length. Elevation difference may not necessarily be the maximum elevation difference within the watershed.

8. Soils Soil of the watershed determines the amount of water that percolates, runoff and silt which will be washed down the valley. Erodibility and transportation of a soil depends on different soil characteristics.

9. Vegetative Cover Vegetative cover is an important landscape element in any watershed. The type of vegetation and its extent of vegetation on watershed lands affect the infiltration, water retention, runoff production, erosion, sedimentation and the rate of evaporation. Vegetation intercepts rainfall, impedes overland flow and promotes infiltration. All of these factors reduce the quantity of runoff to streams. Vegetation binds and stabilizes soil, thereby reducing the potential for erosion.

10. Vegetation also stabilizes stream banks and provides habitat for aquatic and terrestrial fauna. Vegetation functions to slow runoff and reduce soil compaction, allowing better percolation of rain into soil and groundwater recharge. In addition, the patterns, sizes and composition of the vegetation affect reduction of soil erosion. Leaves and branches intercept the falling rain and reduce the effect of raindrop splash. The lesser the falling height of the raindrops, the lesser will be its energy to cause splash erosion. Vegetative litter builds up an organic surface that provides protection of the soil layer. Root systems also help to keep soil material stable from moving down slope.

11. Drainage density Drainage density is the total length of all the streams and rivers in a drainage basin divided by the total area of the drainage basin. It is a measure of how well / poorly a watershed is drained by stream channels. It is equal to the reciprocal of the constant of channel maintenance and equal to the reciprocal of two times the length of overland flow. Rivers that have a high drainage density will often have a more "flashy' hydrograph with a steep falling limb. High drainage density indicates a greater flood risk.

12. 2. Climatic Characteristics The greatest factor controlling stream flow is the amount of precipitation that falls in the watershed as rain or snow. However, not all precipitation that falls in a watershed flows out and a stream will often continue to flow where there is no direct runoff from recent precipitation.

13. Climate parameters like intensity and frequency of rainfall, temperature, humidity, wind velocity and direction affects watershed functioning. It regulates factors like soil properties and vegetation of the region. In the same way, the vegetation type of a region depends totally on its climate type. It is estimated that the total energy of the raindrops equals 100 HP on one ha land during the one cm rainfall for one hour. Thus, all climatic factors determine the amount of surface runoff and transportability of soil particles.

14. 3. Land Use Pattern Watersheds maintain the health of forests, fisheries, wetlands, coastal resources, agricultural landscapes, habitat and local communities. Protecting watersheds ensures sustainable environment that supports recreational activities and a healthy economy. Type of land use, its extent and management are the key factors which affect watershed behaviour.

15. Judicious land use by users is of vital importance to watershed management and functioning. Change of land use within the watershed, especially within the variable source area, greatly affects the collection capacity and consequent runoff behaviour of the watershed. If the land use changes are local, then the impact of such changes is especially apparent in the storm hydrograph.

16. 4. Social Status of Inability Common contributors to water pollution are nutrients and sediment which typically enter stream systems after rainfall washes them off poorly managed fields. These types of pollutants are considered nonpoint sources of pollution because the exact point where the pollutants originated cannot be identified. Such pollutants remain a major issue for water ways because of the difficulty to control their sources hinders any attempt to limit the pollution. Point source pollution originates a specific point of contamination such as if a manure containment structure fails and its contents enter the drainage system.

17. 5. Organization The success of any WSD programme is depends on the type of organization. This is the crucial factor for watershed development. Land use questions can only be tackled in close collaboration with the owners and the local participatory inhabitants. To augment such interaction, the size of the watershed should not be too large or too small. Peoples participation is only effective when a watershed have an organized development agency.

18. Thank You 18 Rainfed Agriculture & Watershed Management Mr. Anil Swami