34 WATERSHED MANAGEMENT

J.S. Rawat

Keywords

Denudation, Cryostatic Pressure, Felsenmeer, clasts, Frost Shattering, Sheeting, Exfoliation Domes, Insolation Weathering, Oxidation and Reduction, Collide Plucking, Hydration, Chelation

    1.1 Definition

Watershed is a natural geomorphic unit. It may be defined as the area which contributes water to a particular stream or sets of streams. Defined by topographic divides, it is an area of land which drains water, sediment and dissolved materials to a common outlet. Thus, watershed is an area that drains surface water to a common outlet. Watershed provides opportunity to estimate the amount of erosion because measurement of river flow and by knowing the area of the watershed and by assuming a diversity of material, rate of land erosion over the whole catchment may be deducted. Watershed is a limited convenient and usually clearly defined and unambiguous topographic unit, available in a nested hierarchy of sizes on the basis of stream ordering. It provides best way to measure precipitation and solar radiations inputs and outputs of discharge. Watershed is ecologically and geomorphologically a relevant management units and its analysis provides a practical analytical framework for spatially-explicit, process-oriented scientific assessment that provides information useful to guiding management decisions.

1.2 Watershed Nomenclature

The nomenclature related with watershed are diagrammatically illustrated in Figure 1. The boundary between two watersheds is referred to as a watershed boundary or water divide. The watershed area is normally defined as the total area flowing to a given outlet or pour point or mouth of the watershed (Fig.1). The pour point is the point at which water flows out of a watershed. This is the lowest point along the boundary of a watershed. The cell in the source raster is used on the pour point about which the contributing area is determined. The source cell may be a feature such as a stream gauging station, dam site or watershed mouth for which characteristics of the contributing area is determined. Watershed has stream or network of streams of different orders having specific flow directions. The confluence of two streams is known as stream junction. Other dimensions of watershed are, it has length, width, perimeter and area (Fig.1).

1.3 Watershed Delineation

Delineation of watershed is the first step to proceed further on integrated watershed modeling and management. There are two ways of watershed delineation. These are, the traditional way through topographic sheets and automated watershed delineation using GIS technology.

1.3.1 Traditional Way –The traditional way of watershed delineation is delineating watershed boundary through topographic sheets. Through this method watershed boundary is drawn manually on a topographic map using pattern of contours (Fig.2A). Delineation of watershed or the total runoff contributory area to a part depends on the watershed drainage pattern. The person who draws the boundary uses topographic features on the map to determine where a divide is located. For watershed delineation at 1:25000 scale maps are suitable and the boundary for a particular watershed is drawn as a line which surrounds all the drainage lines and depressions in the watershed and passes through the highest points between the stream and adjacent one.

Fig.3: Watershed delineation: by automated watershed delineation by pour point technique.

1.3.2 Automated Watershed Delineation –Today GIS software are used to delineate watershed boundary automatically through computer. Through this technique one can generate watershed boundary in a fraction of time. Watershed boundary through computer is determined by using Digital elevation model (DEM) as data input. A stream network can be derived from the DEM.

     From DEM, there are two ways for watershed delineation. These are point based and watershed area wide.

Point-Based Watershed– From the DEM based stream network delineation of individual watersheds can be done based on point of interest such as gauging site, dam site or watershed mouth. Using these points as pour points, area of individual watershed may be delineated (Fig 3).

Area wide watersheds– the area wide watershed for each steam section is delineated by selecting threshold values in terms of area or cells. A large threshold value will have less but larger watersheds. Figure 4 illustrates the determination of area wide watershed delineation at different threshold values. The derivation is based on a threshold accumulation value in terms of area, i.e. 500 hectare (Fig. 4 above) and 50 hectare (Fig. 4 below).

Fig.4: Watershed delineation: by automated by threshold values,i.e.,500 hec (above) and 50 hec (below).

2.0 WATERSHED: AN OPEN SYSTEM

The famous geomorphologist W.M. Davis treated the river is like the vein of leaf, broadly viewed, it is like entire leaf. Like a leaf, watershed is an open system (Fig.2). Close systems are those which possess clearly defined boundaries, across which no import or export of materials or energy takes place. The open system requires a continuing energy supply and is, the effect, maintained by constant supply and removal of energy. The watershed is an excellent example of geomorphological system.

2.1 Energy Inputs and Outputs of Watershed

The watershed has energy inputs to regulate its system. Therefore, the watershed can be envisaged and receiving energy or input from the climate over the watershed surface, and it is loses energy (or output) through the water discharge, sediment and mineral flow through watershed mouth, and evaporation and transpiration from its surface. The endogenic forces below the watershed surface are the second source of energy input of the watershed. The endogenic forces are basically responsible for developing initial form of a watershed. On the initial landform, various kinds of denudational process originated by climatic energy, i.e., rainfall and temperature, the landforms are developed in watersheds of different forms and characteristics. The energy is transformed from one watershed system to another through hydrological and denudation processes from its common outlet, i.e., mouth or through its surface (Fig.5). The environment, ecology and the form of the watershed is a function of all these energy inputs and watershed responses (outputs).

Fig.5: Watershed as an open system (after, Rawat,1985)

3.0 WATERSHED CHARACTERISTICES

The energy provided by the climate is regularized within the watershed system which is controlled by watershed characteristics, i.e., geology, soils, relief morphometry, drainage morphometry, morphology, and vegetation etc. Geological variability in the watershed causes different rates of discharge and silt delivery. Under identical rain inputs, the different rock units of the watershed may produce water discharge and silt delivery at different rates. Soil depth and texture have direct influence on rainwater infiltration, soil moisture storage and groundwater recharge. The relief morphometric parameters such as altitudinal zones (Fig.6), slope and aspect play an important role in controlling both hydrological and denudation process. The aspect play very important role in the distribution of temperature.

The drainage morphometric parameters drainage density, stream frequency, bifurcation ratio, drainage texture, watershed shape, stream order and watershed size play significant role in controlling various watershed processes. Watersheds having high drainage density, stream frequency, bifurcation ratio and drainage texture are subject to high overland flow, low infiltration, low water balance and high rates of denudation. The shape of the watershed influences the time taken for water from the remote part of the watershed to arrive at the outlet. Thus, the occurrence of the peak and the shape of the hydrograph are affected by the watershed shape. Stream ordering (Fig.7), watershed size and hierarchy (Fig.8) also play a significant role in regularization of climatic energy inputs within the watershed.

Morphologically a watershed is constituted of three major zones, i.e., the crest zone, the mid-crest zone and the valley zone. These are also known as upland area, mid land area and low land areas. These morphological zones are characterized by different natural landforms of different genetics, i.e., pluvial, fluvial etc., depending upon the location of the watershed. Vegetation has direct control on hydrological processes such as interception loss, infiltration, overland flow, rainsplash erosion, sheetwash erosion and mass wasting processes. In view of the significant controlling roles, it is necessary to study in detail about the watershed characteristics for wise management of watersheds.

Fig.6: Digital elevation model showing relief of a watershed.

Fig.7: Stream ordering system of a watershed.

Fig.8: Watershed hierarchy of different order.

4.0 WATERSHED INPUT AND OUTPUT BALANCE

The environment and form of a watershed is function of interactions of watershed input parameters (i.e., climate and tectonic processes) with watershed characteristics (i.e.,geology, soils, morphometry, morphology and vegetation etc.). Balance or equilibrium between input and output parameters of a watershed means no environmental deterioration, pollution or healthy environment. An equilibrium state is one in which the many parameters of a watershed system are dynamically balanced. When the entire watershed remains in physical balance, nutrients like water and soils are detained longer within watershed. The soil build and retain more moisture further retaining water and simultaneously, encouraging life. The built-up soils have increased nutrients storage sites and the biogeochemical cycle looks more, keeping the chemicals as richer lingering life (Warshall, 1976). In a balance watershed system, a hill slope profile is product of a balance between runoff, infiltration and erosion. All of these are modified by the degree of soil developed since that in turn controls that grows on hillside and thus controls runoff, infiltration and erosion (Curry,1976).

In disequilibrium state of watershed runoff intensity usually increases, thus, causing erosion on hill slopes. Simultaneously, flood waters reach watercourses faster due to denser and more effective integration of ephemeral channels on the hill slopes, and flood heights increase coupled with increased sedimentation. Increased sedimentation and increased flooding lead to increased lateral erosion of stream channels that must change their channel geometry to accommodate a greater percentage of sediment load to water discharge. Under such disequilibrium state, a headwater anthropogenic change can effect a change in the shape of the whole watershed below it (Curry,1976).

5.0 WATERSHED MANAGEMENT

5.1 Objective and Approach

The broad objective of watershed management is promotion of the overall economic well being and the social improvement of the people within watershed along with maintenance of the watershed. The focus of the watershed management activities is the enhancement of the viability and quality of rural livelihood support systems. To achieve these objectives and fulfill the focus of watershed management, the watershed managers should be well aware about the watershed characteristics, watershed input and output variables, and about the status of watershed input and output interactions. Managing the watershed means healing the wounds and treatment of various types of land diseases and improvement of the quality of the inhabitants. Before starting treatment (i.e., management) of a disease, operation research is needed to find out its cause(s) and effects in the entire system. Then only the disease can be rooted out from the entire system.

To ensure the best use of resources in watershed, the approach of watershed management should be – a) preventive, to control the deterioration of existing relationships between the use of natural resources within the system, and b) restorative, for restoring sustainable relationship which had been destroyed due to actions in past. For this purpose various watershed management practices are used which are described in brief in the following paragraphs.

6.0 WATERSHED MANAGEMENT PRACTICES

For wise management watershed, massive soil and water conservation treatment practices are required. Morgan (1986) has suggested various soil and water conservation practices which are divisible into three major groups. These are related to soil management practices, crop management practices and soil erosion control measure. A brief account of these practices and measures based on Morgan (1986) is presented in the following paragraphs.

6.1 Soil Management

The soil management practices aim to maintain the fertility and structure of the soil. The fertile soils not only give good yield, they also minimize the erosive effects of raindrops, runoff and wind. Soil fertility thus also seen as the key to soil conservation. The various soil management practices used so far in different areas, can be divided into three groups. These are

– using organic matter and  various tillage practices.

6.1.1 Organic contents – Use of organic materials improve the cohesiveness of the soil, increases its water retention capacity and promotes a stable aggregate structure. The major organic materials which may be added are green manures, straw or as a manure which has already undergone a high degree of fermentation. The green manures have high rate of fermentation and yield a rapid increase in soil fertility. Straw decomposes less rapidly and so takes longer to affect soil stability.

6.1.2 Tillage Practices –Tillage is an important soil management technique. The standard tillage operations maintain soil fertility and retain their stability. Tillage provides a suitable seed bed for plant growth and helps to control weeds. The tillage practices which are used for soil management may be divided into two different types. These are – conventional tillage and conservation tillage.

Conventional Tillage – Ploughs made of wood being used over the years is the traditional implement of tillage. Ploughs invert the plough furrow and lift and move all soil in plough layer usually to a depth of 15 to 30cm. Disc cultivator is another traditional equipment for tillage. With disc cultivators the soil is broken up by the passage of saucer-shaped metal discs mounted on axles.

Conservation Tillage – For all types of soils the conventional practices are not useful. For examples, fine sandy soils; very heavy sticky soils; and on structure less soils. On fine sandy soils particularly when dry, conventional tillage practice tend to produce a large number of failure planes, pulverize the soil near the surface and create a compacted layer at plough depth which reduces infiltration and results in increased runoff. The soil is then rapidly eroded by water and on drying in to a fine dust, also by wind. Thus whilst tillage can improve the coarse structures on heavy soils, it can destroy the structure on non-cohesive soils. Under such soils to overcome these problems, tillage operations are restricted either by cutting down on their on their number by carrying out as many operations as possible in one pass, as with mulch tillage and minimum tillage, or by concentrating them only on the rows where the plant grows and leaving the inter row areas untilled, as with strip-zone tillage.

6.2 Crop Management

Plant itself play significant role in reducing soil erosion. Because of differences in their density and morphology, plants differ in their effectiveness in protecting the soil from erosion. In designing soil conservation strategy based on agronomic measures, raw crops must be combined with protection-effective crops in a logical cropping pattern. The agronomic measures for soil conservation are based on the role of plant cover in reducing erosion by various practices such as by crop rotation, cover crops, strip cropping, multiple cropping, high density planting and mulching.

6.2.1 Rotation – The simplest way to combine different crops is to grow them consecutively in rotation. The frequency with which raw crops are grown depends upon the severity of erosion. In low erosion areas, they may be grown every other year, but in the erodible areas, they must be grown only in five or seven years. Suitable crops for use in rotation are legumes and grasses. These provide good ground cover, help to maintain or even improve the organic status of the soil, thereby contributing to soil fertility, and enable a more stable aggregate structure to develop in the soil.

6.2.2 Cover Crops–Cover crops are grown as a conservation measure either during the off-season or as ground protection under trees. They are grown as winter annuals and after harvest, are ploughed in the form of green manure. Ground cover are grown under tree crops to protect the soil from the impact of water drops falling from the canopy.

6.2.3 Strip-Cropping –under strip cropping, raw crops and protection effective crops are grown in alternating strips aligned on the contour or perpendicular to the wind. Erosion is largely limited to the row-crop strips and soil removed from these is trapped in the next strip downslope or dpwnwind which is generally planted with a leguminous or grass crop. This practice of soil management is best suited to well drained soils because the reduction in runoff velocity, if combined with a low rate of infiltration on a poorly drained soil, can result in waterlogging.

6.2.4 Multiple Cropping –The multiple cropping practice involves either sequential cropping, growing two or more crops a year in sequence, or intercropping, growing two or more crops on the same piece of land at the same time. The purpose of this practice is practice is to increase the production from the land whilest providing protection of the soil from erosion.

6.2.5 High Density Planting– By increasing the density of plants, the intensity of soil erosion can be mitigated. This practice is used to obtain the same effect for a monoculture crop that multiple cropping achieves with two or more crops. The high density planting increases infiltration capacity of soil and controls overland flow, consequently, erosion is controlled.

6.2.6 Mulching –To protect the soil from rainsplash and sheetwash erosion, mulching practice is used. Mulching is the covering of soil with crop residues such as straw, maize stalks, palm fronds etc. Such cover control raindrop impact and reduces the velocity of runoff and wind. Mulching also reduces soil temperatures and increases soil moisture. This practice is very useful for semi-humid tropical areas to increase the yield of coffee banana and cocoa.

6.2.7 Revegetation – To control process of erosion on landslides, rilling, gullied, road and other construction sites, sand dunes and mine spoil areas, vegetation plays a significant role. Revegetation is also necessary in the deforestated areas and in areas cleared by patch-cutting.

Various methods and measures are described by Morgon (1986) for restoration of gullied land, restoration of landslide scars and restoration of pastures.

6.2.8 Agroforestry-To control soil erosion, land which is not suitable for agriculture such as along river banks, on terraces and contour bunds, on areas being revegetated to control erosion as windbreaks, and shade trees may be used for planting within a farming system.

6.3  Soil Erosion Control

Mechanical field practices are used to control the movement of water and wind over the soil surface. Studies have suggested several mechanical techniques to reduce the velocity of runoff and wind, increase the surface and groundwater water storage capacity or safely dispose the excess water. Some of such important mechanical measures are contouring, contour bunds, terraces, waterways, stabilization structures, windbreaks and geotextile (Morgan,1986).

6.3.1 Contouring– Carrying out ploughing, planting and cultivation on the contour can reduce soil loss down to 50% compared with cultivation sloping land by –and-down the slope. The effectiveness of contour farming varies with slope steepness and slope length, for it is inadequate as the soil conservation measure for lengths greater than 180m at 10 steepness.

6.3.2 Contour Bunds –Contour bands are earth banks of 1.5 to 2m wide, built across the slope to act as a barrier to runoff from a water storage area on their upslope side and to break up a slope into segment shorter in length than is required to generate overland flow.

6.3.3Terraces – Terraces are earth embarkments constructed across the slope to intercept surface runoff and convey it to a stable outlet at a non erosive velocity and to a shorten slope length. These terraces can be classified into three main types: diversion terraces, retention terraces and bench terraces. Diversion terraces intercept overland flow and channel it across the slope to suitable outlet. Retention terraces are used where it is necessary to conserve water by storing it on the hillside. Bench terraces consists of a series of alternating shelves and risers and are employed where steep slopes, up to 300 need to be cultivated.

6.3.4 Waterways – The purpose of waterways in a conservation system is to convey runoff at a non-erosive velocity to a suitable disposal place. Normally its dimension must provide sufficient capacity to confine the peak runoff from a storm with a ten year return period. Three types of waterways can be incorporated in a complete water disposal system. These are diversion channels, terrace channels and grass waterways.

6.3.5 Stabilization Structures – For gully reclamation and gully erosion control stabilization structures play an important role. Small dams, usually 0.5 to 2.0 m in height, made from locally availavle materials such as earth, wooden plank, brushwood or loose rock, are built across gullies to trap sediment and thereby reduce channel depth and slope.

6.3.6 Windbreaks –To control the erosive winds, shelterbelts placed at right angle are very useful to reduce wind velocity. The shelterbelts are spaced at regular intervals to break up the length of open wind blow. Inert structures such as stone walls, salt and brushes fences and cloth screens can be used to perform similar functions on a smaller scale. A shelterbelt is designed so that it rises sharply on the windward side and provides both a barrier and a filter to wind movement.

6.3.7 Geotextile – Several types of netting woven from natural fibers such as jute or made from artificial fibres such as nylon are used in erosion control. They are supplied in rolls, unrolled over the hillslope from the top and anchored with large pins or stapled. They are designed to give temporary stability on road sides and on steep slopes not used for agriculture until such time as the vegetation cover grows.

7.0 WATERSHED INSTRUMENTATION FOR MONITORING

Because watershed provides opportunity to estimate the amount of erosion and water balance through measurements hydrometeorological parameters and by knowing the area of the watershed and by assuming a diversity of material, rate of land erosion and water balance over the whole watershed may be deducted before watershed treatment, during watershed treatment and after watershed treatments. Therefore, to define the responses of various management activities in the watershed, watershed instrumentation is an essential part of watershed management. For this purpose two permanent monitoring stations within the watershed are needed. First, to monitor watershed responses or outputs, i.e., water discharge and sediment and solute flow, a hydrological station at the mouth of the watershed and second to monitor climatic input parameters, i.e., rainfall, temperature, evaporation, wind velocity etc, a meteorological station within the same watershed (Fig.9) is required.

Fig.9: Watershed instrumentation for monitoring watershed inputs and their responses,

(A) Hydrological Station at the mouth of the watershed –see Fig.10 and (B) Meteorological Station within the watershed –see Fig.11).

Fig.10: View of a hydrological station at the mouth of the watershed consisting of rectangular weir and water level stage recorder..

   7.0 HISTORY OF WATERSHED MANAGEMENT IN INDIA

Watershed Management projects has been launched by the Government of India from early 1970s onwards. Different watershed management programmes like DPAP (Draught Prone Area Progress), DDP (Dessert Development Program), RVP (River Valley Project), NWOPRA (Nainital Watershed Development Project for Rain-fed Areas) and IWDP (Integrated Watershed Development Programme) were launched subsequently in venous environmental regions, these were consistently being affected by water stress and draught like situation.During mid 1980s and early 1990s emphasized was given to the integrated watershed development programme with participatory approach. This approach had focused on rising crop productivity and livelihood improvement in watersheds.

In 1974, Government of India constituted a committee under the Chairmanship of Prof. C.H. Hanumatha Rao. This Committee recommended new guidelines in 1975 which emphasized on collective action and community participation, including participations of primary stakeholders through community based organizations, non-government organizations and Panchayati Raj Institutions (PRIs).The watershed development programmes were reviewed and revised again in 2001 and develop guidelines known as Hariyali Guideline to make further simplification and involvement of PRIs more meaningful in planning, implementation, evaluation and community improvement.

In the year 2005, the Heeranchal Committee evaluated the entire Government sponsored NGo and donor implemented watershed development programmes in India and suggested a shift in focus away from a purely engineering and structural focus to a deepest concern with livelihood issues. The major objectives of the watershed management programme are:

1. 1. Conservation, up-gradation, and utilization of natural endowments such as land, water, plant, animal and human resources in a harmonious and integrated manner with low cost, simple, effective and replicable technology.
   2. generation of massive employment reduction of inequalities between irrigated and rain-fed areas and poverty alleviation.

References

• Currey,  R.R.  (1976) watershed form and processes the elegant balance,  the Co evolution, quarterly issue No. 12.
• Morgon, R.P.C.(1986): Soil Erosion and Conservation, Engllish Language Book Society/Longman, pp.1-298.
• Rawat, J.S (1987): Modelling of water and sediment budget: concept and strategies, CATENA Supplement 10, A Cooperative Journal of the International Society of Soil Science, West Germany, pp.147-159.
• Warshall, J.P. (1976):Straining wisdom, co-Evolution, Quarterly issue, No. 12, pp.4