Soil and water conservation measures in watershed areas 14.1 In situ soil moisture conservation practices Storage of rainfall or rain water at the place where rainfall occurs for its effective usage is known as in situ moisture conservation. This can be achieved by different measures. Improving the soil surface conditions to increase infiltration of rainfall and reduction of runoff are the two basic requirements in dry lands. Hence land configuration determines the ease with which water can enter the soil. The different in situ moisture conservation practices which result in changed land configuration are as follows. 14.1.1 Ridges and furrows: The field must be formed into ridges and furrows. Furrows of 30-45 cm width and 15-20 cm height are formed across the slope. The furrows guide runoff water safely when rainfall intensity is high and avoid water stagnation. They collect and store water when rainfall intensity is less. It is suitable for medium deep to deep black soils and deep red soils. It can be practiced in wide row spaced crops like cotton, maize, chillies, tomato etc. It is not suitable for shallow red soils, shallow black soils and sandy/ gravelly soils. It is not suitable for broadcast sown crops and for crops sown at closer row spacing less than 30 cm. Since furrows are formed usually before sowing, sowing by dibbling or planting alone is possible. 14.1.2 Tied ridging: It is a modification of the above system of ridges and furrows wherein the ridges are connected or tied by a small bund at 2-3 m interval along the furrows to allow the rain water collection in the furrows which slowly percolated in to the soil profile 14.1.3 Broad bed furrows (BBF): This practice has been recommended by ICRISAT for vertisols or black soils in high rainfall areas (> 750 mm). Here beds of 90-120cm width, 15 cm height and convenient length are formed, separated by furrows of 60 cm width and 15 cm depth. When runoff occurs, its velocity will be reduced by beds and infiltration opportunity time is increased. The furrows have a gradient of 0.6%. Crops are sown on the broad beds and excess water is drained through number of small furrows which may be connected to farm ponds. It can be formed by bullock drawn or tractor drawn implements. Bed former cum seed drill enables BBF formation and sowing simultaneously, thus reducing the delay between receipt rainfall and sowing (Fig 14.1). Broad bed furrow has many advantages over other methods. • It helps in moisture storage • Safely dispose off surplus surface runoff without causing erosion • Provide better drainage facilities • Facilitate dry seeding • It can accommodate a wide range of crop geometry i.e. close as well as wide row spacing. • It is suitable for both sole cropping and intercropping systems. • Sowing can be done with seed drills. Fig. 14.1 Alternate crop and row arrangement on broad beds 14.1.4 Dead furrows At the time of sowing or immediately after sowing, deep furrows of 20 cm depth are formed at intervals of 6 to 8 rows of crops. No crop is raised in the furrow. The dead furrows can also be formed between two rows of the crop, before the start of heavy rains (Sep – Oct). It can be done with wooden plough mostly in red soils. The dead furrows increase the infiltration opportunity time 14.1.5 Compartmental bunding Small bunds of 15 cm width and 15 cm height are formed in both directions to divide the field into small basins or compartments of square or rectangular shape of 6 x 6 m to 10 x 10 m size using bund former .They are useful for temporary impounding of rain water which facilitates high infiltration resulting in high moisture storage in the soil. Recommended for black soils with a slope of 0.5 to 1%. Maize, sunflower, sorghum perform well in this type of bunding (Fig 14.2). Fig 14.2 Compartmental bunding 14.1.6 Scooping Scooping the soil surface to form small depressions or basins help in retaining rain water on the surface for longer periods (Fig 14.3). They also reduce erosion by trapping eroding sediment. Studies have shown that runoff under this practice can be reduced by 50 % and soil loss by 3 to 8 t /ha. Fig. 14.2 Scoops for insitu moisture conservation 14.1.7 Inter plot water harvesting Water is drawn from part of a small catchment and used in lower portion for crop production. There may be 1: 1 cropped: catchment area or 1:2 catchment: cropped area. 14.1.8 Zingg terracing or conservation bench terracing These are developed by A.W.Zingg, in USA. Zingg terracing is practiced in low to medium rainfall areas in black soils with contour bunds. It is a method of land shaping where lower one third portion of the land adjacent to the contour is leveled to spread to the runoff water coming from the remaining two third portion of the field .This rainfall multiplication technique ensures at least one good crop in one third area even in low rainfall years. Usually during medium rainfall years water intensive crops (like paddy) are cultivated in the levelled portion (receiving area) while dry crops are cultivated in the unlevelled (donor) area. 14.2 Mechanical / Engineering measures of soil conservation When Agronomic measures alone are not adequate, mechanical measures are to be adopted to supplement the agronomic measures. Mechanical measures usually involve construction of mechanical barriers across the direction of flow of rainwater to retard or retain runoff and thereby reduce soil and water loss. The mechanical measures include: – Contour bunding – Graded bunding – Bench terracing – Gully control / plugging – Vegetative barriers etc. A bund or terrace is an earthen embankment or a depression or a combination of both constructed across the land slope to control runoff and minimize soil erosion by reducing the length of slope. By reducing the slope, the velocity of runoff is not allowed to attain critical value, which initiates scouring. 14.2.1 Contour bunding It is most popular in the country. Contour bunding consists of narrow based trapezoidal bunds on contours to impound runoff water behind them so that it can gradually infiltrate into the soil for crop use. Contour bunding is generally recommended for areas receiving <600 mm rainfall (low rainfall areas) and for permeable soils up to slopes of about 6%.Spacing between two bunds is commonly expressed in terms of the V.I.(vertical interval) which is the difference in elevation between two similar points on two consecutive bunds. The following formula is used for determining spacing of bunds. V.I..= S/a + b where, V.I.= vertical interval (m) between consecutive bunds S = % slope of land ‘a’ and ‘b’ constants depends on soil and rainfall characteristics The height of the contour bunds depends on slope of land, spacing of bunds and maximum intensity expected. In deep black soils, contour bunds have been a failure due to cracking of bunds during dry months and water stagnation above the bunds for prolonged periods during rainy season (Fig. 14.4). Fig 14.4 Contour bunding 14.2.2 Graded bunding: Graded bunds or channel terraces are constructed in high rainfall areas of >600 mm where excess water has to be removed safely of the field to avoid water stagnation. In case of highly impermeable soils like deep black soils graded bunds are recommended even in lesser rainfall area (500 mm) as in case of Bellary region of Karnataka. Water flows in graded channels constructed on upstream side of bunds at non-erosive velocities and is led to safe outlets or grassed waterways. Channel portion of the graded bunds is put under cultivation and the grassed waterways are permanently kept under grass. 14.2.3 Bench terracing: Bench terracing is practiced in steep hill slopes, where mere reduction of slope length is not adequate for reducing the intensity of scouring action of runoff flowing down. In addition to slope length reduction, the degree of slope is also reduced. Bench terracing consists of transforming relatively steep land into a series of level strips or platforms across the slope to reduce the slope length and consequently erosion. The field is made into a series of benches by exca
vating soil from upper part and filling in the lower part of terrace. It is normally practiced on slopes > 14% i.e. from 16 to 33%. Depending on soil, climate and crop requirements bench terraces may be table top or level, sloping outwards or sloping inwards (Fig. 14.5). Type of bench terrace Suitability Table top or level terraces Suitable for medium rainfall areas (750mm) with even distribution and highly permeable deep soils. Sloping outwards terraces Constructed in low rainfall areas (<750 mm) with permeable soils of medium depth. Sloping inwards terraces Constructed in heavy rainfall areas (>750 mm) and soils with poor infiltration rate. Fig 14.5 Types of bench terraces 14.2.4 Gully control: Gullies the result of sheet and rill erosion left unchecked. The basic approach to gully control involves reduction of peak flow rates through the gully and provision of channel for the runoff water. Agronomic measures of soil conservation like contour cultivation, strip cropping, cover crops, mulching etc., aid in reducing the peak flow rates through gullies. The provision of the stable channel for the flow that must be handled is accomplished by stabilizing the gully sides and bed by establishing vegetation. Temporary structures such as brush check wood dams, loose rock dams, rock fill dams and woven wire dams, and permanent structures such as chute spill ways, drop spill ways, concrete check dams and pipe spill ways are practiced for reducing channel gradient to maintain velocities below erosive level (Fig. 14.6 and 14.7). Fig. 14.6 Concrete check dam for gully control Fig. 14.7 Loose boulder check dam for gully erosion control 14.2.5 Vegetative barriers: These are the rows of closely planted grass or shrub along the contours for erosion control in Agricultural lands. They check the velocity of runoff and retain the sediment by acting as barrier to runoff. Khus Khus (Vetiveria zyzynoides) is the most recommended plant for this purpose. Grassed waterways: These are drainage channels either developed by shaping the existing drainage ways or constructed separately for effecting drainage of agricultural lands. They are used to handle runoff, discharge from graded bunds, broad base terraces and bench terraces. Objectives of grassed waterways 1. To provide drainage to agricultural lands 2. To convert unstable channels or gullies into stable channels by providing grass cover 3. For leading water at non erosive velocity into farm ponds Grassed water ways are normally dug to a shallow depth of 0.15 to 0.5 m. They are constructed one or two seasons ahead of the construction of channel terraces. 14.3 Forestry Measures: Forest lands are usually found at higher elevations where the slopes are steepest, soils are less stable and easily eroded and precipitation is heavy. The leaves and branches of trees and shrubs intercept the rain and reduce the impact of raindrops. Contour trenching and aforestation is recommended for improving the productivity of forests. Contour trenching is done by excavating a trench along the contour and forming soil bank. Rain water thus held up in these trenches for some time and facilitate the growth of vegetation. Plants are sown in trenches taking advantage of water (Fig. 14.8 and 14.9 ). Tree species suitable are Pinus patula, Pinus kesia, Acacia nilotica, Eucalyptus camaldulensis etc. Fig. 14.8 Regenerating degraded land with contour trenching in Anantapur, A.P Fig . 14. 9 Continuous contour trenching 14.4 Agrostological Measures: Grasses prevent erosion by intercepting rainfall and by their binding power of the soil particles. A grass-legume association is ideal for soil conservation. Legumes build 67up soil fertility by fixing atmospheric N in root nodules. Grasses have several uses in soil conservation like: – Stabilizing the surfaces of water ways, contour bunds and front faces of bench terraces – Stabilizing the gully slopes and sides – Preventing wind erosion The desirable characters of grasses for soil conservation are: o Should be perennial o Drought resistant o Rhizomniferous o Develop good canopy o Deep root system o Prostate growth habit o Less palatable to cattle o Useful for cottage industries Useful grasses: Cenchrus ciliaris, Chloris guyana, Cynodon dactylon, Dicanthium annulatum, Heteropogan contortus, Iseilema laxum, Panicum antidotale Legumes: Atylosia scarbaceoides, Centrosema pubescence, Stylosanthus hamata, Grass + legume Cenchrus ciliaris + Stylosanthus hamata, is best for A.P.