Soil Erosion and Watershed Management: USLE, Terraces and Bunds, Waterways, Gully Control, Spillways, Earthen Dams and Water Harvesting

The Universal Soil Loss Equation predicts long-term average annual sheet and rill erosion: A = R · K · L · S · C · P, with A the soil loss (t/ha/year in metric use). The factors are defined relative to a unit plot — 22.13 m long on a uniform 9 per cent slope, kept in continuous clean-tilled fallow up and down the slope — for which L, S, C and P are all 1.

Worked: R = 500, K = 0.3, LS = 1.2, C = 0.4 and P = 0.5 give A = 500 × 0.3 × 1.2 × 0.4 × 0.5 = 36 t/ha/year. Because the equation is a product, it is also used backwards — to find the largest C (the crop system) or P (the practice) that keeps A below the tolerable soil loss: with R = 600, K = 0.25, LS = 1.5 and P = 0.6, holding A at 10 t/ha/year needs C ≤ 10/135 = 0.074. The USLE does not predict gully erosion or deposition, and single-storm estimates use its modified form (MUSLE) with a runoff term in place of R; RUSLE revises its factors.

Biological (agronomic) measures work by cover and roughness: contour farming, strip cropping (contour, field and buffer strips), mulching, cover crops, crop rotation, conservation tillage, agroforestry and grassed buffers. Engineering (mechanical) measures shorten the slope length and store or safely convey runoff: contour bunds (level earthen embankments along contours, for low-rainfall areas with permeable soils, where water is to be held), graded bunds (laid on a gentle longitudinal grade to drain excess water safely in higher-rainfall areas or less permeable soils), and terraces.

Bench terraces convert a steep slope into a flight of level or nearly level steps and are the standard treatment for cultivated hill slopes. Their geometry is fixed by the vertical interval VI (the drop between benches) and the land slope S per cent: the horizontal interval is HI = 100 VI/S, and with vertical risers the bench width equals HI; a sloping riser of U horizontal to 1 vertical takes U × VI of it. A 1 m vertical interval on a 20 per cent slope gives 5 m benches. With vertical risers and cut balanced against fill, each bench has a triangular cut and fill of height VI/2 over half its width, so the earthwork per metre length is (W/2)(VI/2)/2 = W·VI/8, and there are 10 000/W metres of bench per hectare: the earthwork is V = 1250 × VI m³/ha, independent of the bench width — 1500 m³/ha for a 1.2 m interval.

Vegetative (grassed) waterways carry runoff from terraces, graded bunds and field outlets down the slope without eroding their own bed. They are shaped parabolic (the most natural and stable form), trapezoidal or triangular, sized by the peak rate from the rational method, and designed with Manning’s equation for a permissible velocity that depends on the grass cover and soil — the grass is what allows a velocity that bare soil could not take. For a parabolic section of top width T and depth D, the area is A = (2/3)TD: a 6 m by 0.5 m waterway has A = 2 m² and at a design velocity of 1.25 m/s carries 2.5 m³/s.

A gully is stabilised by stopping the water that feeds it at the head, reducing the grade of its bed and establishing vegetation. Temporary structures — brushwood, loose-rock and woven-wire check dams — trap sediment and slow the flow long enough for vegetation to take hold. Permanent structures are spillways that lower water safely through a fall in grade:

• Drop spillway: a weir (straight or box inlet) over which water falls vertically into a stilling basin that dissipates its energy; for drops of up to about 3 m where the gully bed is stable below.
• Drop-inlet (pipe) spillway: a vertical riser feeding a conduit through an embankment, so that water is stored upstream and released in a controlled way; as the head rises the flow passes from weir control at the riser crest to orifice control and finally to full pipe flow.
• Chute spillway: an open, steep, lined channel with an inlet, a conveyance section and an energy-dissipating outlet (such as the SAF stilling basin); for large drops and high flows.

Earthen dams for farm ponds and small reservoirs are homogeneous (one soil), zoned (an impervious core with pervious shells) or diaphragm type (a thin impervious membrane). They fail by overtopping (inadequate spillway or freeboard), seepage and piping (water finding a path through or under the dam), and sloughing or slope failure (steep or saturated slopes, rapid drawdown). Seepage is analysed through the phreatic line, which in a homogeneous dam with a horizontal filter is the Kozeny parabola: with h the water depth and d the horizontal distance from the filter face to where the parabola meets the reservoir (with Casagrande’s correction), its focal parameter is y₀ = √(h² + d²) − d, and the seepage per unit length is q = k·y₀. A 12 m head with d = 16 m gives y₀ = 20 − 16 = 4 m, and with k = 10⁻⁶ m/s, q = 4 × 10⁻⁶ m³/s per metre.

A watershed is the area draining to a common outlet, and it is characterised by its size, shape, drainage network, slope, soils and land use. Shape indices compare it with a circle or square: the form factor R_f = A/L_b² (L_b the basin length), the circularity ratio (basin area over the area of a circle of the same perimeter) and the elongation ratio; a compact, circular basin gives a sharper, earlier peak than an elongated one. The drainage network is ordered by Strahler’s scheme — unbranched streams are order 1, two streams of order n join to make order n + 1 — and summarised by the bifurcation ratio R_b = N_u/N_(u+1) (typically 3 to 5) and the drainage density D_d = ΣL/A, the total stream length per unit area; high drainage density means impervious soils and a flashy response. A basin of 50 km² and 10 km length has a form factor of 0.5; 120 km of streams in 60 km² is a drainage density of 2 km/km².

Land-use capability classification groups land into eight classes by the severity of its limitations (slope, erosion hazard, depth, wetness, salinity, climate). Classes I to IV are suitable for cultivation, with limitations and the need for conservation practices increasing from I (few limitations) to IV (very severe, occasional cultivation only); classes V to VII are unsuited to cultivation and are kept under permanent vegetation — pasture, grazing and forestry; class VIII is fit only for wildlife, recreation and water supply. Subclasses name the kind of limitation (e for erosion, w for wetness, s for soil, c for climate).

Water budgeting closes the balance over a watershed for a period: P = Q + ET + ΔS + G (precipitation equals runoff, evapotranspiration, change in soil and surface storage, and deep percolation), and tells how much can be harvested without starving downstream users. Rainwater harvesting is in situ (contour furrows, ridges, micro-catchments that hold rain where it falls) or ex situ (collecting runoff into tanks, ponds and recharge structures); the harvestable volume is runoff coefficient × rainfall × catchment area — 0.2 × 0.6 m × 5 ha (50 000 m²) = 6000 m³. Check dams across drainage lines hold water, trap silt and recharge groundwater; farm ponds — dugout or embankment type — store runoff for life-saving irrigation, and are sized from the runoff volume, with allowances for seepage and evaporation losses and a spillway for the excess.