Gully Erosion and Land Degradation
7. Gully erosion and land degradation
Land degradation due to gully erosion (by water) occurs when the interaction power of the rainfall energy and overland flow exceeds the resistance of soil to point of detachment (Hadley et al., 1985). There are three process includes are
1. Detachment of part of land with soil due to rainfall force (de Jong, 1994)
2. Transportation of detached sediment (dispersion on the landscape) by surface run off
3. Deposition of soil in low lying sites and aquatic ecosystems (Eswaran, 2001)
Formation of gullies start when runoff hits bare soil. The energy of the water scours away the soil under vegetation. When the vegetation and topsoil are removed, gullies spread rapidly up and down drainage lines until there is insufficient runoff to continue the erosion. New gullies can form quickly, but once they stabilise they lose very little soil. Gully erosion occurs on most bare soils, but the most vulnerable soils are, where the soil structure disperses or collapses when wet where below the soil surface, gully formation begins as tunnel erosion in such cases. Other vulnerable soils are those developed from granitic or sandstone rocks (NSW, 2018).
Soil erosion caused by runoff water is one of the most important land-degradation processes in the world (Jahantigh and Pessarakli, 2011). Rate of gully extension reported that with an average loss of 2.35 million m3 of agricultural soil per year at Jawa Block, Rewa District in Madhya Pradesh, India and the area will increase by 50% in the next 50 years. Gully erosion is a result of the interaction of land use, climate and slope. It occurs on many different soils and landforms (Savindra Singh and Sheo Prakash, 1987).
In the lateritic highland of Paschim Medinipur District, West Bengal and also in the Kansai and Silabati river catchments area (Anonymous, 2018), gullies remove top soil rendering the soil unfertile and destructs the plants grown in affected area by reducing soil available for rooting and for storing water. It is a common form of soil erosion causing siltation, sedimentation in the valley floor. Sub-soil erosion reduces soil productivity and the extension of gully in the monsoon season threatens roads, building, trees etc. This erosion damage depends on infiltration, slope pattern, shear stress for erosion, vegetation cover as well as anthropogenic factor. Rate of erosion increases due to deforestation on highland region and other areas. Physical and cultural landscape is changing with the advancement of gully in agricultural land, forested land and other useful lands. Along the southern banks of Silabati River and in the northern banks of the Kangsabati River, land degradation is increasing by the rapid extension of gully.
The degradation has been assessed in 2006 and 1990, which indicates that degradation of land is more in 2006 than 1990 which indicates that this
area has undergone severe gully erosion during these years. The study was also conducted to measure the landscape change by preparing two different FCC using Landsat-TM image. Results suggests that the degradation or conversion of land from good land to bad land during 1990 to 2006. Recent study on cross sectional measurements of different gully channel during 2006 and 2009 at Gangani shows the erosional and depositional status and retreat of gully head. A residual relatively hard soil is lying on the re-recent valley floor. It is also indication of differential erosion. It is observed that erosion towards gully head very high due to massive erosion by overland flow retreating every rainy season in Rangamati gully basin along Kangsabati. The attached roads also destroyed in many portions or completely cut off in Rangamati gully basin. Damaging of house, falling of buildings and trees and cracks of roads into gully sites are common in the erosion affected areas of Paschim Medinipur District.
Undulating and sloppy upland are having sandy loam type of texture and are fallow are more prone to severe erosion. In the forest areas gullies occur and it becomes wider and denser. In the bare land, the loose and soft type of soil is very susceptible to erosion. Rangamati including the northern bank of Kangsabati River, Salboni, Garhbeta, Jhargram, and Lalgarh are highly affected by gully and rill erosion. Land-use and land-cover map of the area prepared for two years 2000 and 2010 from Landsat TM image, which showed that, in 2000, degraded forest was less due to more vegetation coverage, erosion has taken place in the northern bank of Kangsabati River and in the southern bank of Silabati River. In 2010, degradation of land is higher than 2000. The major eroded part is the Silabati and Kangsabati River. Degradation of forest occurs due to continuous gully and rill erosion in the area. Study was done to know the soil fertility status in gully lands which indicated reduction in soil fertility due to the removal of top soils from the exposed soil surfaces. Varied field measurements showed that bare exposed surfaces near and around the gullies are crusted which further reduces infiltration and seed growth therefore exposure of bare surfaces must be avoided in the in Gangani gully basin. Scattered vegetation cover is the main factor of the gully and rill development in this area.
Pani (2016) reported that Lower Chambal Valley (Fig. 4) is one of the most degraded regions in India. Kumar and Pani (2013) studied the impact of soil erosion of the region lower Chambal valley. They reported that region is severely affected by gully and ravine erosion (Fig. 5) and degraded land is increasing at an alarming rate. The impact of land degradation is through its impact on agricultural system (Pani et al., 2011). Crop productivity of the region has adversely affected by land degradation. Due to long and
continuous fluvial erosion fertile land has gone out of plough which also increased the water table depth.
Mohita (2018) reported that, the aerial survey conducted for Chambal development scheme depicted that area occupied by ravines up to 4.5 to 6.0 metre depth is about 50,600 hectares. In Madhya Pradesh about 4 to 8 lakh hectares are affected by deep gulling and ravines along the banks of rivers Chambal (Fig. 3) and Kali Sindh. Many gullies and ravines occur in the upper catchment areas of the Kangsabati River, Purulia district, West Bengal. River courses of the Ganga, the Gandak, the Kosi and the Son are affected by ravines in Bihar. The Shiwalik range also is affected by Gully erosion. Rivers descending from the Shiwalik hills and flowing through Punjab are locally called as Chos. Hoshiarpur district of Punjab affected by Chos. Out of 130 km of the Shiwalik, around hundred streams debouch on to the plains. In foothills of lower Shiwaliks, land degradation is a serious problem due to Soil erosion by water. Nearly 20% of the total land in the region is under gullies (Kukal and Sur, 1992). Over-exploitation and mismanagement of soil resources through deforestation, over-grazing and clearance of land for agricultural purposes disregard to slope and topography of the land lead to Ecological degradation in Shiwaliks. To prevent gullies in the Shiwaliks, efforts are being made by several agencies but with little success (Kukal, 2009).
Fig 4: Land degradation in Chambal River in Madhya Pradesh, India
Type A: Deep (Young) Ravine
Type B: Moderate (Mature) Ravine Type C: Shallow (Old) Ravine Fig 5: Network of gullies and types of ravines (Pani and Carling, 2013) A study has been conducted to know the causes and impacts of the gully erosion menace in the Auchi community of Edo State, Nigeria (Afegbua et al., 2016). Many factors bordering on anthropogenic and natural causes involves heavy rainfall, geology, nature of the soil, deforestation, lack of proper drainage systems, lack of tangible erosion control measures were the major causes of the gullying in the Auchi gully sites, which led to death of people and destruction of settlements. Natural sources resulting in formation of gullies are more severe than anthropogenic ones. Field observations in this study showed that local geology, run-off as encouraged by the residents, soil porosity and loose sands, groundwater table at the sites, were major contributors to the rates and magnitude of gullying in parts of Auchi community (Anonymous, 2010).
Mansur (2014) studied the Gully Erosion in Dutse Sahelian Zone of Jigawa State, Nigeria. It was noted that there was rapid enlargement of the gullies and a drip of discharge flowing down the gully was also reported.
The gullies have influenced soil losses and water about 3.15 and 2.10kg m year-1 of soil losses was observed from the gullies due to heavy rainfall during the previous raining season and associated rate of sediment production caused by gully erosion. However, the menace of gully erosion caused a severe damage to farm lands, routes of communication and settlements.
A study has undertaken in Central Ethiopian Highlands (Addisu et al., 2013). It was observed that rilling and gullying are the major physical land degradation features in the site. The road constructed in 1999-2004, between Addis Ababa to Ambo and to Fiche in the highlands of Northern and Western Shoa (Central Ethiopia), caused gulling and other road drains. The road influences a concentration of surface runoff, a diversion of runoff to other catchments, and an enhancement in catchment size, which are the main
reasons for gully development after road construction. Therefore, techniques should be used to spread concentrated runoff in time and space and to encourage its infiltration instead of directing it straight onto unprotected slope areas.
Gully erosion plays an important role for sediment production in several southern African catchments. The distribution of gully erosion clearly indicates that this process should be included in the calculation of sediment yield, where the surficial material (saprolite) is highly vulnerable to erosion.
Nevertheless, in the USLE, gully erosion is completely neglected (Marker and Sidorchuk, 2003).
Many types of erosion processes degrade soil, air, and water quality, including irrigation-induced erosion, ephemeral gully erosion, classical gully erosion, stream bank and bed erosion, tillage translocation erosion, wind erosion, and sheet and rill erosion. Ephemeral gullies are defined as small channels eroded by concentrated flow that can be easily filled by normal tillage, only to reform again in the same location by additional runoff events (Soil Science Society of America, 2008). Ephemeral gully erosion forms due to concentrated flow of surface runoff along a channel as well as by subsurface flow by seepage and flow through preferential pathways. Studies show that ephemeral gully formation is very common on cultivated land, particularly in conventional tillage systems without support practices. It can be controlled through conservation plans with practices viz., grassed waterways, terraces, vegetative barriers etc. (USDA, 2018).
In Indian Lateritic Terrain of Birbhum district, West Bengal, the inherent characteristics of lateritic landscape combined with human intervention are responsible for rill and gully erosion risk. Moderate erosion risk is more in the area and it is related to the lateritic landscape affected by rill erosion. On the contrary both rills and gully networks in basins have severe erosion risk (V.C. Jha, and S. Kapat, 2009).