Increasing Water Storage by Reducing Runoff and Increasing Infilteration under Rainfed Condition

Author: Mohan Prakash Shrestha Institute of Agriculture and Animal Science, Tribhuwan University, Nepal INCREASING WATER STORAGE BY REDUCING RUNOFF AND INCREASING INFILTRATION UNDER RAINFED FARMING Infiltration refers to the movement of water into the soil layer. The rate of this movement is called the infiltration rate. If rainfall intensity is greater than the infiltration rate, water will accumulate on the surface and runoff will begin. Improvements in soil conditions and soil-water regime to optimize crop production can be accomplished by runoff management techniques which vary with the situation, depending on existing conservation problems, on soil and on ecological region. In the arid and semi-arid regions, the choice of management is clear; all rainfall must be retained by techniques that reduce storm-water runoff, improve infiltration and increase the water storage capacity of the soil. In the humid and sub-humid areas, a balance has to be struck between conservation of soil and water by runoff control and the avoidance of surface water logging, so the options are not as straight forward. In general, runoff is best minimized by ensuring high infiltration of rainwater into the soil through biological conservation measures. Where this cannot be done to full effect, particularly in areas of high-intensity storms or where there are periods of poor crop cover, earth works (physical control measures) can provide surface protection by holding water giving it time to soak through the surface. Such physical conservation measures involve land shaping, the construction of contour bunds, terraces and ridges. These require considerable technical design, supervision, proper construction and maintenance. In contrast, the biological methods include some soil management and agronomic cultural practices that is normally the companion of profitable agriculture such as appropriate land use and preparation, fertility maintenance, crop residue management, the use of cover crops and appropriate crop husbandry. Much research on rainfall-runoff management and its influence on erosion and soil-water conservation have been accumulated over the years. Some of the successful practices are briefly described in the following section. Mechachanical Measures a. Levelling Leveling reduces the slope of the land, thus reducing the speed of the runoff and ultimately increasing the infiltration. The agricultural operations also can be carried out easily in leveled land. A plain wooden structure called maddim is used for leveling ploughed land in High Himalayan region of Nepal. A heavy stone is put on the maddim for increasing the pressure required for leveling. Sometimes, a man also may sit in place of stone. Such an indigenous technology for leveling is called planking. With this practice, there is a very good seed soil contact and very good germination of the crops. Secondly, there is moisture conservation in the field. Small clods are pressed and broken into finer particles and this way soil structure is improved. b. Bunding Bunding is the construction of small embankment or bunds across the slope of land. Bunds act as a barrier to runoff. Bunds breaks the slopes into segments which shorter in length than required to overland flow. The reduction of slope not only reduces the soil erosion but also retain the runoff water in surrounded area of the bund. They control the effective length of slope and thereby reduce the gain in velocity of runoff flow to avoid gully formations. Bunds are constructed with the following objectives: • To increase the time of concentration of rainwater where it falls and thereby allowing rainwater to percolate into the soil • Converting a long slope into several ones as to minimize velocity and thereby reducing the erosion by runoff water • To divert runoff either for water harvesting purposes Types of bunds 1. Contour bund Contour bunds are earth banks, 1.5 wide, thrown across the slope along the contour. They are suitable in low rainfall areas (up to 600 mm annual rainfall) having slopes 1-70 and more permeable soils while not suitable in clay soils. The main function of these bunds is to reduce slope length which in turn reduces soil erosion and to store and retain water. 2. Graded bunds When grade is provided in bunds it is referred to as grade bunds. It is used in areas having rainfall greater than 700 mm per year. However, it can be used in low rainfall areas having heavy texture soil. In such areas, when rainfall takes place, a portion of water is ponded over the surface, to remove such water some grade should be provided. It requires establishment of grassed waterway as an outlet for disposal of surplus water. The main functions of graded bunds are to reduce slope length to control soil erosion and to dispose the excess water to the suitable outlet. c. Terracing Terracing is the method of modifying land surface for erosion control and water conservation. Terracing involves construction of embankments or ridge and steps like structure across the slope. Effective length slope is reduced by terracing. From experiment, it has been found that soil loss is directly proportional to the slope length of power 0.5. According to this, soil loss is increased with increase in slope length. Thus, terracing reduces erosion by reducing slope length. In Nepal, it has been found that mean annual soil erosion rates from level bench terrace was 20 ton ha-1 while erosion rates from similar unterraced land was 100 ton ha-1. Functions of terraces:  Intercept surface runoff and convey it to suitable outlet at non erosive velocity.  Reduce the slope of length by splitting the slope length in different parts.  Traps the splashed soil particles. Types of terraces 1. Diversion terraces Used to intercept overland flow on a hillside and channel it across slope to a suitable outlet, e.g. grass waterway or soak away to tile drain; built at slight down slope grade from contour. Diversion terraces are further classified as:  Mangum type: Formed by taking soil from both sides of the embankment.  Nichols type: Formed by taking soil from upslope side of the embankment only.  Broad-based type: Bank and channel occupy width of 15m.  Narrow-based type: Bank and channel occupy width of 3–4m. banks have steeper slope which cannot be cultivated. To make cultivation possible bank should not exceed 140 slope if small machinery is used or 8.50 for large machinery. 2. Retention terraces Level terraces; used where water must be conserved by storage on the hillside. This is only recommended for permeable soils on slopes less than 4.50. 3. Bench terraces Alternating series of shelves and risers used to cultivate steep slopes. It can be constructed where steep slope up to 300, need to be cultivated. Risers are vulnerable to erosion and protected by vegetation and faced with stones or concrete. Various modifications can be made to permit inward-sloping shelves for greater water storage or protection on very steep slopes or to allow cultivation of tree crops or market-garden crops. d. Contouring Contouring of slopy land is an ethno-engineering practice for soil conservation in west Himalayan region of Nepal. Farmers have developed this technology for cultivation of slopy lands by constructing terraces comprising of plots and sub plots by using small stones. Stone wall fencing is also constructed for individual land holding. Terracing of slopy land helps in conserving soil and moisture and prevents soil erosion. This also checks the surface run off. Contour trenches Contour trenches are used both on hill slopes as well as on degraded and barren waste lands for soil and moisture conservation and afforestation purposes. The trenches break the slope and reduce the velocity of surface runoff. It can be used in all slopes irrespective of rainfall conditions (i.e., in both high and low rainfall conditions), varying soil types and depths. e. Sub Soiling Sub soiling is a term used in agriculture. It’s just the process of deep tilling of the ground (12”-18’ deep), in an effort to increase the subsoil’s ability to absorb water. It is mainly used to incompact the soil, but it also improves the aeration of soil and dispersion of nutrients. It originally got its name because top soil is usually only 6” deep at the most, then below, that is the sub soil. Tilling deeper than 6” will incorporate the sub soil with the top soil. The layer of dense clay will accelerate erosion on hill sides since the clay will not absorb water as fast it needs to be during natural rainfall. Thus, the top soil becomes water logged and heavier and will slip off the sub soil. The same sub soil and thin top soil on flat land can create boggy mess in pasture and fields. Therefore, sub soiling or the loosening of the sub soil without bringing it to the surface increases the percentage capacity of water of the soil stirred. It decreases the capillary conducting power of the soil stirred. The purpose of mechanical sub soiling is loose soil, breaking the plow pan, increase the speed and amount of rainfall infiltration, reduce runoff, reduce water evaporation losses. Sub soiling is only due to mechanical scarification, not tilling the soil, so that top soil does not work after the chaos, a small amount of ground-breaking, it is particularly suitable for shallow black soil. Cultural Measures a. Deep Ploughing Plough operations help to keep the upper soil layers porous at least for a short time especially in compact soils that restrict root development and infiltration. Ploughing can help to minimize runoff by assisting infiltration. To be effective over a longer term, the soil aggregates must be stable and resistant to breakdown under raindrop impact. A wide range of special tillage operations involving soil inversion, chiseling, sub soiling or deep tillage (for soils with an impeding layer within rooting depth) have been found to be beneficial in improving surface detention, storage, infiltration, root development and by minimizing soil hardening (Ofori and Nandy 1969; Ofori 1973; Lal 1984; Ogunremi et al. 1986). The effectiveness of plough in soil and water conservation also depends on the conditions under which it is carried out and its frequency. Recent studies have emphasized the importance of cultivating only when soil conditions, particularly the soil moisture content are right to avoid structural breakdown and smearing (Davies 1974; Spoor 1975). The pulverizing effect of conventional tillage can be minimized by reducing the number of operations on the land. this can be achieved by cultivating only the small strips of land required for seedbeds thus leaving wide untilled zones (strip zone tillage); by carrying out tillage with a mulch retained on the ground (mulch tillage) or completing as many activities as possible in one pass (minimum tillage) as with plough-plant operations b. Contour Farming Contour farming involves aligning plant rows and tillage lines at right angles to normal flow of runoff. It creates detention storage in the soil surface horizon and slows down the runoff thus giving the water time to infiltrate into the soil. The effectiveness of contour farming in water and soil conservation depends not only on the design of the system but also on soil, climate, slope aspect and land use. The beneficial effect is least marked on compact or slowly permeable soils because these become saturated quickly compared to highly permeable soil. Contour bunds are another important physical measure. These are earth banks, 1.5 to 2 m wide, forming buffer strips at 10 to 20 m intervals across the slope. Although little reduction in runoff occurs, there is considerable reduction in soil loss as demonstrated by Roose (1967) in West Africa. c. Mulching Mulch farming is a system of maintaining a protective cover of vegetative residues such as straw, maize stalks, palm fronds and stubbles on the soil surface at all times. The system is particularly valuable where a satisfactory plant cover cannot be established at the time of year when erosion risk is greatest. The beneficial effects of mulching include protection of the soil surface against raindrop impact, decrease in flow velocity by imparting roughness, and improved infiltration capacity. It also enhances burrowing activity of some species of earthworms (e.g. Hyperiodrilus spp. and Eudrilus spp. (Lal 1976)) which improves transmission of water through the soil profile (Aina 1984) and reduces surface crusting and runoff and improves soil moisture storage in the root zone. These effects have been widely reported. That mulch effectively reduces soil loss has been shown in both field (Borst and Woodburn 1942; Lal 1976) and laboratory (Lattanzi et al. 1974) studies. Lal (1976) reports an annual saving of 32% of rainfall in water runoff from mulching in humid Western Nigeria. Roose (1988) (Table 2) reports drastic reductions in runoff and erosion from a mulched pineapple field on a 20% slope. The quantity of mulch required for maintenance of favorable infiltration capacity and structural stability depends on the rate of residue decomposition, climate, soil properties, and relief and rainfall characteristics. Studies relating soil loss to bare ground indicate that about 70% of the soil surface must be covered by mulch to be effective and for straw, the optimum rate of mulch appears to be 4 to 6 t/ha-1 in the tropics, though improvements in soil physical properties have been observed up to a mulch rate of 12 t/ha-1 (Lal 1976). Crop residue mulch has also been known to beneficial as well when ploughed in or when brought in after the soil has been ploughed. Favorable effects of mulching on soil and water conservation have been reported similarly for the arid and semi-arid regions (Lawes 1966; Djokoto and Stephens 1961; Adeoye 1985). Adequate or durable mulching material is less readily available in these regions with their long dry season. There are however other techniques providing utilizable mulch by using cover crops, alley cropping and no tilla Residue mulch effect on runoff and soil erosion Country Runoff (% rainfall) Erosion (t ha-1 yr-1) Bare Mulched Bare Mulched Ghana 49.8 1.4 313 0.42 Côte d'Ivoire 36.4 0.33 18.3 1.9 Nigeria 29.0 0.1 410 1.0 Mensah-Bonsu and Obeng (1979); Roose (1988); d. Strip Cropping Strip cropping is the practice of growing different crops in alternate strips across the slope such that they serve as vegetative barriers to erosion. The alternate strips consist of close growing erosion resisting crops to erosion permitting crops like row crops. Erosion is limited to row crop strips and the eroded materials is trapped on the erosion preventing strip planted to grasses or legumes. It is important that planting be rotated so that the strip planted to row crops this year will be planted to protection effective crops the next year. For controlling water erosion, the strips are always on the contour but in dry regions strips are placed across to the prevailing wind direction for wind erosion control. The strip crops reduce the velocity of surface runoff and force them to infiltrate into the soil thereby facilititates to the conservation of rain water. The strip cropping controls erosion in two ways:  By slowing down of runoff water flows through the close growing strip and  By increasing infiltration rate which reduces total runoff volume. Types of Strip Cropping 1. Contour strip cropping Contour strip cropping consists of growing alternate strips of erosion permitting and erosion resisting crops along the contour. It is adopted on the level land across the slope instead of up and down hill, for checking the flow of surface water. From field studies, it has been observed that strip cropping on the contour plays a key role in conserving the soil and water when combined with terracing. The width of strips will vary depending upon the topography. 2. Field strip cropping In field strip cropping, the strips are made in more or less parallel, across the slope in uniform width without taking into consideration the exact contours. This method is useful on regular slopes and with soils of high infiltration rates. 3. Buffer strip cropping In buffer strip cropping, the strips of grasses or legume crops are laid between contour strip crops in regular rotation. The strips are meant to take care of critical slopes (steep or highly eroded) in fields under contour strip cropping. 4. Wind strip cropping The strips of uniform width are laid out at right angles to the direction of prevailing wind without regard to contour. e. Alley Cropping Alley cropping is an agro-forestry system integrating trees and shrubs with annual food crop production (Kang et al. 1981; Vegara 1982). In this system arable crops are grown in the spaces between rows of planted woody shrubs or trees, which are pruned during the cropping season to provide in situ green manure and to prevent shading of crops. The beneficial effects of the system in reducing erosion, surface runoff and soil moisture loss depend on the proper choice of the protective species. Promising results have been obtained in alley-cropping arable crops (such as maize) with Gliricidia and Leucaena. The ecological compatibility of this system is yet to be established for the different eco-regions. f. Cover Cropping The practice of growing grasses and legumes to cover the ground surface to control erosion from water and wind is called cover cropping. Cover crops are grown as a conservation measure either during the off-season or as ground protection under trees. Planted cover crops such as Mucuna pruriens utilis, Pueraria phaseoloides, Centrosema pubescens, Setaria spp., Stylosanthes spp. and Glycine spp. provide another technique of achieving in situ mulch. Fallowing with suitable cover crops conserves soil water (Pereira et al. 1958) improves water use efficiency, weed control and soil organic matter. It is the most satisfactory method of building up the organic content of soil. Wilson (1979) reported that dry weight of residue from cover crops can be as much as 6.5 to 11 t/ha-1 yr-1. The effectiveness of cover crops in soil and water conservation however depends on species characteristics including ease of establishment, vigor of growth, depth of rooting, rapidity of establishment of surface cover etc. Fallowing for one or two years with Mucuna pruriens utilis, Pueraria phaseoloides, Centrosema pubescens has been reported to improve soil structure and infiltration capacity (Pereira et al. 1958; Kannegieter 1967; Okigbo and Lal 1977; Lal et al. 1978). In many areas, cover crops are grown as winter annuals and ploughed to form the green manure prior to the sowing the main crop. Crops used as cover are rye, oats, mustard, hairy vetch, sweet clover, crotalaria, winter peas, cow peas etc. To be effective cover crops, crops must be  Quick to establish and spread rapidly.  Provide good canopy cover  Aggressive to suppress the weeds  Deep root system to improve macro porosity of soil  Hardy and drought tolerant Cover crops are grown as ground cover under tree crops to protect soil from the impact of water drops falling from canopy. They are particularly important with tall crops where the height of fall is sufficient to cause the drops to approach their terminal velocity. The objectives of cover crops are:  To protect the surface of the soil from being splashed with raindrop.  To build up soil organic matter and improve its physical properties.  To suppress the weed growth and reduce management cost.  To maintain changes in the microclimate and in soil temperature thereby providing a better environment for crop growth. g. No-tillage Farming Sound soil management is essential to the success of soil conservation schemes because it ensures good plant cover. No-tillage involves a method of seeding through crop residue mulch by opening a narrow slot in the soil for seed placement without mechanical or secondary tillage operations. Chemical weed control is used. The beneficial effects of no-tillage in soil and water conservation are widely recognized in humid and sub-humid climates where the system seems to have a broad application. The benefits include soil moisture conservation due to reduction in storm runoff, improved infiltration capacity, enhanced earthworm activity, and reduced evaporation loss. It also reduces soil erosion and maintains organic matter content at high levels. As much as a 5-fold reduction in runoff has been reported under no-tillage compared to conventional tillage (Lal 1976). The effectiveness of no-tillage farming in soil and water conservation is improved when used in association with planted cover crops. h. Ridge and Mound Tillage The ridge-furrow system is a commonly used physical conservation practice. Ridge-and-furrow systems when aligned parallel to the contour lines have the dual purpose of erosion control and surface drainage. Their advantages are greater, the less steep the terrain and the more permeable the soil. Fournier (1967) reported a 7 to 13-fold decrease in erosion and runoff in parts of West Africa due to ridging (Table 4). Ridge-tying is also an effective soil and water conservation system especially in arid and semi-arid regions. Mounds and tied mounds are also effective in conserving water and soil. The effectiveness of tied-ridges depends on soil, slope, rainfall and design characteristics. Tied ridging on clay soils may induce water logging which may be followed by mass movement (Gray and Brenner 1970). In severe storms, poorly designed ridge-furrow systems may fail the row catchments can over-top and the water flow unimpeded down the slope with the danger of it accumulating enough energy to scour and transport soil. Generally, for small landholders with only hand implements or animal traction and low-value subsistence crops, the ridge-furrow system used along the contour is a satisfactory method of enhancing infiltration and reducing runoff. Tying ridges or mounds to permit more rainwater to infiltrate is an effective system in drier areas (< 1000 mm annual rainfall) on gentle slopes (< 7%) but not in wet years or more humid areas. Other useful practices for small farmers include contour planting and strip cropping. For large-scale mechanized farming, additional devices to cope with large and exceptional runoff are necessary. These include: terraces and contour bunds to break up long slopes; and diversion channels and watercourses to dispose safely of unavoidable runoff.