Erosion and runoff management in hilly, foot hills and valley lands; processes and factors affecting them
Soil erosion due to water is a major concern in all the northern hilly states of India in general and Uttarakhand in particular. The peculiar terrain condition and high rainfall in the state pose a serious problem with regard to soil erosion. The often indiscriminate destruction of forests and woodlands leads to terrain deformation and accelerates soil erosion.
Soil loss through water erosion
In Uttarakhand, quantitative soil loss was estimated using USLE through derived information on factors of erosivity (R), soil erodibility (K), topography (LS), cover and management (C) and conservation practice (P). The computed soil loss in the state has been categorized into six classes of erosion hazards, namely very slight (<5 tonne ha–1 year –1 ), slight (5–10 tonne ha–1 year –1 ), moderate (10–15 tonne ha–1 year –1 ), moderately severe (15–20 tonne ha–1 year –1 ), severe (20–40 tonne ha–1 year –1 ), and very severe (40–80 tonne ha–1 year –1 ). Figure 6 and Table 10 show the spatial distribution of estimated soil loss in Uttarakhand. Table 11 shows the district-wise distribution of soil loss status of the state. It is observed that about 500,000 ha (9.36%) and 309,000 ha (5.78%) area is affected by very slight and slight soil loss respectively, covering mainly Pauri Garhwal, Nainital and Udham Singh Nagar districts. The moderate and moderately severe classes occur in Pauri Garhwal, Naintal, Champawat and Udham Singh Nagar districts with an area of 394,000 ha (7.37%) and 359,000 ha (6.71%) respectively. The severe and very severe erosion classes are found mainly in the districts of Dehradun, Uttarkashi, Tehri Garhwal, Rudraprayag, Chamoli and Bageshwar with an area of 473,000 ha (8.84%) and 1,750,000 ha (32.72%) respectively.
Soil loss tolerance This is a limit which denotes the maximum level of soil erosion that will permit crop productivity to be sustained economically and indefinitely. Considerable work has been done on this aspect and tolerance limits ranging from 4.5 to 11.2 tonne ha–1 year –1 have been reported40. Soil loss in excess of 11.2 tonne ha–1 year –1 affects the effectiveness of water conservation structures. This stage leads to gully formation which in turn obstructs the cultural activities34 . Analysis shows that moderate, moderately severe, severe and very severe classes cover an area of 394,000 (7.39%), 359,000 (6.71%), 473,000 (8.84%) and 1,750,000 ha (32.72%) of the state TGA and exceed the tolerance limit of 11.2 tonne ha–1 year –1 . It is also evident that surface soil is lost every year from different regions of the state leading to a huge amount of nutrient loss in a year. In terms of fertilizer loss, this accounts for a staggering loss of nutrients in a year. Hence, it is evident from the analysis that there is an urgent need to adopt appropriate soil conservation measures in Uttarakhand.
Soil and water conservation
The Himalayan region in general and Uttarakhand in particular are home to many varieties of flora and fauna, where biodiversity still continues to exist in abundance. However, this land of rich biodiversity has its problems too. Severe soil erosion is one such problem which poses a serious threat to sustainability. Moreover, the region, being one of the most hazard-prone belts in the Asian continent, is susceptible to earthquakes, landslides and floods. Under these circumstances, adoption of appropriate soil and water conservation measures is necessary to conserve biodiversity and sustainability. Conservation measures can have only a short-term impact, unless they are accompanied by objectives to tackle the underlying causes. Thus, the knowledge of the causes of soil erosion and adoption of appropriate conservation measures is essential for sustainable productivity in the state.
Causes of soil erosion Soil erosion in Uttarakhand is both due to anthropogenic activities and natural causes, and is continuing over the years with varying intensities. Besides, anthropogenic activities like uncontrolled deforestation and unscientific landuse, including shifting cultivation have accelerated the process of soil erosion. The major causes of erosion in the state could be attributed to weak geological formation, active seismicity and deforestation.
Weak geological formation and active seismicity
The geological formation of the entire northern region of India, including Uttarakhand is weak and unstable. Geotectonic movements make the land mass unstable, resulting in landslides and mass movements. The soils of this region are developed on stratified soft sedimentary and tertiary rocks, which are also susceptible to erosion. During monsoon, the incessant rains or cloud bursts make these soils malleable and easily detachable, resulting in solifluction and sheet erosion down the slopes.
During field survey massive deforestation activities were noticed in Uttarakhand. Incidents of widespread deforestation and mining in many parts of the state are common. Very good dense forests are being converted into poor stock, thin degraded fallow lands. Considerable areas under forest have been brought under agriculture in the recent past. Deforestation and forest degradation are the leading causes of water erosion in undulating and steep-sloping hills. The situation has been aggravated further due to high population pressure, demands for fuel and timber, etc.
Suggested conservation measures There is need to develop site-specific strategies and resource conservation techniques to preserve soils, production potential, sustain productivity, conserve in situ rainwater, minimize soil erosion, mitigate droughts, moderate floods downstream, harvest and recycle inevitable run-off and ensure environmental security.
Agronomic measures Agronomic measures on cultivated lands are recommended in mildly sloping areas (1%–6%) with the objective of maximizing conservation of in situ rainfall for sustained and higher production. Contour farming, i.e. up and down cultivation is generally practised by farmers for the sake of convenience, which facilitates run-off water to attain higher velocity resulting in more run-off and soil erosion. Farm operations such as ploughing, seeding and interculturing along the contour lines or across the slope help in the formation of natural ridges and furrows, which act as a series of mini barriers and reservoirs to intercept rainwater reducing run-off and soil nutrient loss.
Intercropping allows canopy legumes such as groundnut, green gram, black gram, soybean and cowpea under inter-row spaces of crops like maize, sorghum and castor. It provides adequate cover on the ground and thereby reduces erosion risk apart from biological insurance to increase productivity of rainfed arable lands.
Tillage makes the soil surface more permeable to infiltration of rainwater. This practice also reduces run-off, soil and nutrient losses and enhances crop yields. Conservation tillage by covering 30% of the soil surface with crop residues was found to be effective within the frame work of conservation of natural resources and sustained production.
Mulching is an important agronomic practice that not only prevents soil erosion by dissipating kinetic energy of raindrops but also facilitates infiltration, reduces evaporation and improves soil structure which eventually enhance crop yield. In low-rainfall areas, mulching helps in conserving moisture in the soil profile while in high rainfall areas, it reduces run-off and soil loss, resulting in higher crop yields.
Water Harvesting, Terracing and Other Engineering Structures
Mechanical soil and water conservation measures are required for controlling soil erosion, retaining maximum rainfall within the slope and safe disposal of excess runoff from the top to the foot hills of India. These structures are often used in case of extreme soil degradation. The measures are: Bunding-small earthen barriers built on agricultural lands with slopes ranging from 1%–6% slope. Bunds are used in agriculture to collect surface run-off, increase water infiltration and prevent soil erosion. Graded bunds-constructed in medium to high rainfall areas of ~600 mm year−1 . Contour bunds- either mechanical or vegetative barrier created across the slope. A study conducted at Doon valleys in the northwestern hills region indicted that contour bunds decreased runoff 25%–30% compared to field bunds . Bench terrace and half moon terrace-adopted where soil depth is >1.0 m. Half-moon terraces are level circular beds having 1 to 1.5 m diameter cut into half-moon shape on the hill slopes. Beds are used for planting and maintaining saplings of fruit and fodder trees in horticulture/agro-forestry land uses. Grassed waterways-channels laid out preferably on natural drainage lines in the watershed. Water harvesting ponds-dug-out embankment type of water harvesting structure used for creating seasonal and perennial ponds at the foot of a micro-watershed for irrigation and fish farming purposes.
Intercropping and Contour Farming
Agronomical practices like use of cover crops, mixed/inter/strip cropping, crop rotation, green manuring and mulch farming are vital practices associated with integrated nutrient management. Growing soybean (Glycine max)/groundnut (Arachis hypogoea)/cowpea (Vigna radiata) with maize (Zea mays)/jowar (Sorghum bicolor)/bajra (Pennisetum glaucum) is a common example of intercropping in the drylands . Strip cropping is a combination of contouring and crop rotation in which alternate strips of row crops and soil conserving crops are grown on the same slope, perpendicular to the wind or water flow in drylands and hilly regions, respectively. Intercropping cowpea with maize (2 rows of cowpea with 1 row of maize) decreased runoff by 10% and soil loss by 28% compared to pure maize. Minimum runoff (36% of rainfall) was recorded under barnyard millet (Echinochloa frumentacea L.) followed by black soybean (Glycine max L.) and maize which was 37% and 42%, respectively. Black soybean and maize alone had maximum soil loss of 7.1 and 6.7 ton ha−1 , respectively, followed by barnyard millet (4.8 ton ha−1 ). The practice of line sowing of wheat and mustard (Brassica juncea L.) crops and maintaining row ratio of 8:1 ensured optimum use of space and soil moisture, increased wheat equivalent yield by 14% and net returns by 30% compared to mixed sowing Water use efficiency, yield and net return as affected by different technologies and crop rotation in farmers’ fields of Uttarakhnad, Jammu and Kashmir and Himachal Pradesh.
Watershed Approach Integrated watershed management, which involves soil and water conservation coupled with suitable crop management, is another excellent strategy for mitigating soil erosion. Development and management of watershed resources to achieve optimum production without causing deterioration in the resources base is integrated watershed management. It involves construction of check dams along gullies, bench terracing, contour bunding, land leveling and planting of grasses. These strategies will increase percolation of water, decrease runoff and improve water availability. Several reviews are available on the performance of watershed development projects, as well as their limitations. An operational research project on watershed management at Fakot by the CSWCR&TI during 1975–1986 is a successful example of participatory integrated watershed management approach. Conservation agriculture along with above-said practices has great potential to reverse soil loss.