Watershed Management and Rainwater Harvesting
Watershed Management and Rainwater Harvesting tests watershed geometry and behaviour, the ridge-to-valley treatment approach, and the design logic of harvesting structures. Numerical items usually involve the rational formula or drainage density.
Watershed characterisation
- A watershed is the area draining to a common outlet; stream order increases where two streams of the same order join.
- Drainage density = total stream length / watershed area (km/km²); higher density means quicker, peakier runoff.
- Shape factor, elongation ratio, circularity ratio and bifurcation ratio describe form; time of concentration is the longest travel time to the outlet.
Management and harvesting
- Treat the ridge (trenching, afforestation), the slopes (bunding, waterways) and the valley (check dams, farm ponds) in sequence.
- Rainwater harvesting: roof-water harvesting, farm ponds, percolation tanks, check dams, recharge pits and contour trenches.
- Participatory planning, entry-point activities and user groups decide whether a watershed project survives.
Worked example
Catchment 100 ha, runoff coefficient C = 0.4, rainfall intensity I = 50 mm/h. Using Q = C × I × A/360, Q = 0.4 × 50 × 100/360 = 5.56 m³/s. Reasoning: with A in hectares and I in mm/h, dividing by 360 gives m³/s; the coefficient is the fraction of rainfall that becomes runoff.
Traps
Using a 24-hour rainfall total in place of intensity; wrong units (A in ha, not km²); confusing circularity with elongation ratio; designing a farm pond on capacity alone without a safe surplus arrangement.