Water Resources, Watershed Management and Forest Hydrology

What to remember

• A watershed is the land area that drains to one common outlet. Management works on the "ridge to valley" principle: treat the upper slopes first, then mid-slopes, then the drainage lines and the lower land.
• Forests regulate water more than they create it. They raise infiltration, cut peak flows and erosion, and give clean, steadier dry-season flow, but trees also use water through transpiration and interception.
• Rational formula: Q = C × i × A / 360 (Q in m³/s, i in mm/hour, A in hectares). It is used for small catchments.

Water resources and the hydrologic cycle

The hydrologic cycle is the continuous movement of water between sea, air and land: evaporation and transpiration, condensation, precipitation, interception, infiltration, percolation, runoff and storage.

Water balance of a catchment: Precipitation = Runoff + Evapotranspiration + Change in storage (soil and groundwater). Over a long period, storage change is nearly zero, so P = Q + ET.

Sources of water:

• Surface water: rivers, streams, tanks, lakes, reservoirs.
• Groundwater: water stored in aquifers below the water table, drawn through wells and bore wells.
• Rainwater harvested in structures such as farm ponds and percolation tanks.

An aquifer is a rock or sediment layer that stores and yields usable water. The water table is the upper surface of groundwater in an unconfined aquifer. Recharge is water added to groundwater, mostly by infiltration and percolation.

Rainfall measurement: a non-recording (Symons) raingauge measures daily rain; recording gauges (tipping bucket, float type, weighing type) give intensity. Rainfall is given in mm; one mm of rain on one hectare is 10 cubic metres (10,000 litres).

Watershed concept and classification

A watershed (catchment, drainage basin) is bounded by a ridge line or divide. A large basin is divided into smaller units: sub-basins, watersheds, sub-watersheds, milli-watersheds and micro-watersheds. Project planning is usually done for micro-watersheds so that people and field conditions can be handled.

Watershed characteristics:

• Size and shape: a circular watershed gives a sharp, high peak; an elongated one gives a lower, spread-out peak.
• Slope and relief: steeper slopes give faster runoff.
• Drainage density = total stream length / watershed area; high density means fast runoff.
• Stream order (Strahler): the smallest unbranched streams are first order; two first-order streams join to form second order, and so on.
• Soil, geology and land use decide infiltration.

Components of watershed management

Watershed management is the proper use of land, water and vegetation of a watershed for sustained production while protecting soil and water.

Principles: people's participation; hydrologic unit as planning unit; in-situ moisture conservation (keep rain where it falls); use land according to capability; build structures to harvest runoff; protect and improve vegetation.

In-situ measures: contour trenches, staggered trenches, field bunding, mulching, broad bed and furrow. Ex-situ measures: farm ponds, check dams, percolation tanks, nala bunds, storage tanks.

Programmes (names only): the Integrated Watershed Management Programme (IWMP) merged earlier area programmes for drought-prone areas, deserts and wastelands; the watershed development component now sits under the Pradhan Mantri Krishi Sinchayee Yojana (PMKSY). Andhra Pradesh has run state water-conservation and water-harvesting campaigns such as Neeru-Chettu with community participation; check the latest official release for the current form of any scheme. Kolleru is an inland wetland of Andhra Pradesh that is a Ramsar site.

Forest hydrology

Forest hydrology studies the effect of forest cover on rainfall partition, soil water, streamflow and water quality.

Partition of rainfall in a forest:

• Interception: rain held on leaves and branches and evaporated; it never reaches the ground.
• Throughfall: rain that drips through gaps or from leaves.
• Stemflow: water running down the trunk (small share, but concentrated at the base).
• Net rainfall = Throughfall + Stemflow = Gross rainfall − Interception loss.
• Litter and humus hold water and break the energy of rain.
• Infiltration is high under forest because of root channels, humus and soil fauna.

Effects of forest on water:

• Reduces surface runoff and soil loss; reduces peak flood flow and delays it.
• Supports baseflow (dry-season flow) in many catchments by improving infiltration, though trees also lose water by transpiration.
• Improves water quality: low sediment, low turbidity.
• Removing forest (clear felling, fire) generally increases total water yield and peak flows in the short term, with higher erosion.
• Fast-growing, high-water-use plantations in dry areas can reduce streamflow and groundwater recharge; species and site should be chosen with care.
• Cloud forests collect moisture from fog by fog drip.
• Riparian buffers (vegetated stream edges) filter sediment and stabilise banks.

Evapotranspiration (ET): total loss by evaporation from soil and water and transpiration from plants. Potential evapotranspiration (PET) is the loss when water is not limiting; the Penman method is a standard estimate. A lysimeter measures actual ET directly.

Infiltration is entry of water into soil; percolation is its downward movement through soil. A double-ring infiltrometer measures infiltration rate. The Horton model shows that infiltration rate falls with time to a constant final rate.

Runoff estimation and water harvesting structures

Runoff coefficient (C) is the ratio of runoff to rainfall; it is low for forest and high for roofs and rock.

Rational formula: Q = C × i × A / 360, with Q in m³/s, i in mm/h and A in ha.

Worked example: C = 0.4, i = 90 mm/h, A = 36 ha: Q = 0.4 × 90 × 36 / 360 = 3.6 m³/s.

Runoff volume check: rainfall 50 mm on 10 ha with C = 0.3: rain volume = 50 mm × 10 ha × 10 m³ per (mm·ha) = 5,000 m³; runoff = 0.3 × 5,000 = 1,500 m³.

Common structures:

Silt and sediment: a reservoir loses storage by siltation; catchment treatment by afforestation and soil conservation increases reservoir life.

Water quality and wetlands

Forest streams usually carry less sediment and fewer pollutants. Sediment load, turbidity, dissolved oxygen and nutrients are the common indicators. Wetlands (tanks, marshes, mangroves, lakes) store flood water, recharge groundwater and shelter birds and fish. The Ramsar Convention is the international treaty for wetland conservation.

Floods, droughts and soil moisture

A flood is a high peak flow that overtops banks. Forests cannot stop very large floods from extreme rain, but they reduce small and medium floods and slow the rise. A hydrograph is a graph of discharge against time; a steep, high rising limb shows fast runoff. Baseflow is the slow, groundwater-fed part of stream flow that continues in dry weather. Drought is a long period of rainfall shortage. Soil moisture is held between field capacity (upper limit after drainage) and permanent wilting point (lower limit where plants cannot take up water). The difference is the available water. Organic matter and humus raise the water-holding capacity of soil, so forest soils store more usable water.

Exam traps

• Interception vs throughfall: interception is the loss held on leaves; throughfall reaches the ground.
• Stemflow vs throughfall: stemflow runs along the trunk; both together are net rainfall.
• Infiltration vs percolation: infiltration is entry at the surface; percolation is movement downward inside the soil.
• ET vs PET: PET assumes unlimited water supply.
• In-situ vs ex-situ: in-situ keeps water where it falls; ex-situ stores it elsewhere.
• Percolation tank vs farm pond: percolation tank aims at recharge; farm pond aims at storage for use.
• Watershed vs catchment: the terms are used almost alike; a watershed is the divide-bounded drainage unit.
• Forest and water yield: forests help infiltration and regulation, but do not always increase total water yield.

One-liners

• 1. A watershed is bounded by a ridge line (divide).
• 2. Watershed treatment goes from ridge to valley.
• 3. Net rainfall = throughfall + stemflow.
• 4. Rational formula: Q = C i A / 360 (m³/s, mm/h, ha).
• 5. 1 mm of rain on 1 ha equals 10 m³ of water.
• 6. Drainage density is stream length divided by area.
• 7. Strahler's system classifies streams by order.
• 8. Humus and litter increase infiltration under forest.
• 9. A lysimeter measures evapotranspiration.
• 10. Percolation tanks recharge groundwater.
• 11. Contour trenches are in-situ moisture conservation.
• 12. Kolleru in Andhra Pradesh is a Ramsar wetland.

Practice questions

1. A watershed is bounded by:

   1. A ridge line (divide)
   2. A river mouth only
   3. A district boundary
   4. A road network
   Answer

   A. A ridge line (divide)

   The divide separates drainage to different outlets.

2. Over a long period with no change in storage, the water balance of a catchment is:

   1. P = Q - ET
   2. P = Q x ET
   3. P = ET - Q
   4. P = Q + ET
   Answer

   D. P = Q + ET

   Precipitation leaves as runoff or evapotranspiration.

3. Rain water running down the tree trunk is called:

   1. Interception
   2. Throughfall
   3. Percolation
   4. Stemflow
   Answer

   D. Stemflow

   Stemflow is a small but concentrated part of net rainfall.

4. Net rainfall under a forest canopy equals:

   1. Interception plus stemflow
   2. Throughfall plus stemflow
   3. Gross rainfall plus interception
   4. Throughfall minus stemflow
   Answer

   B. Throughfall plus stemflow

   Net = gross - interception loss = throughfall + stemflow.

5. Evapotranspiration can be measured directly with:

   1. A current meter
   2. A double-ring infiltrometer
   3. A lysimeter
   4. A Symons raingauge
   Answer

   C. A lysimeter

   A lysimeter records water used by plants and soil.

6. The main purpose of a percolation tank is:

   1. Groundwater recharge
   2. Flood storage only
   3. Fish culture only
   4. Silt removal
   Answer

   A. Groundwater recharge

   Water stored in it seeps into the aquifer.

7. Drainage density is calculated as:

   1. Slope divided by stream order
   2. Number of rainy days divided by area
   3. Watershed area divided by stream length
   4. Total stream length divided by watershed area
   Answer

   D. Total stream length divided by watershed area

   High density means many channels per unit area.

8. When two first-order streams join, the stream formed is of:

   1. Zero order
   2. Second order
   3. Third order
   4. First order
   Answer

   B. Second order

   In Strahler's system equal orders add one to the order.

9. Which inland wetland of Andhra Pradesh is a Ramsar site?

   1. Pulicat sandbar only
   2. Nagarjunasagar reservoir
   3. Kolleru
   4. Papikonda hills
   Answer

   C. Kolleru

   Kolleru Lake is the Ramsar-listed wetland in the state.

10. Watershed treatment follows which principle?

    1. Valley to ridge
    2. Ridge to valley
    3. Downstream only
    4. Outlet first, ridge never
    Answer

    B. Ridge to valley

    Upper catchment is treated first, then lower areas.

11. Which method is a standard estimate of potential evapotranspiration?

    1. Horton
    2. Symons
    3. Strahler
    4. Penman
    Answer

    D. Penman

    Penman combines energy and aerodynamic terms.

12. 1 mm of rain on 1 hectare equals:

    1. 1,000 cubic metres
    2. 100 cubic metres
    3. 10 cubic metres
    4. 1 cubic metre
    Answer

    C. 10 cubic metres

    10,000 m2 x 0.001 m = 10 m3.

13. Interception loss is the water that:

    1. Enters the groundwater
    2. Runs down the trunk
    3. Flows in streams
    4. Is held on leaves and evaporates before reaching the ground
    Answer

    D. Is held on leaves and evaporates before reaching the ground

    It never reaches the soil.

14. Baseflow in a stream is mainly:

    1. Flood peak flow
    2. Flow from rainfall just fallen
    3. Slow groundwater-fed flow in dry weather
    4. Flow from snow only
    Answer

    C. Slow groundwater-fed flow in dry weather

    Baseflow continues between rain events.

15. Horton's model shows that infiltration rate:

    1. Is constant from the start
    2. Rises steadily with time
    3. Becomes zero in minutes
    4. Falls with time to a constant final rate
    Answer

    D. Falls with time to a constant final rate

    Soil pores fill and swell, so the rate drops.

16. Contour trenches are an example of:

    1. In-situ moisture conservation
    2. Ex-situ storage only
    3. Reservoir construction
    4. Canal lining
    Answer

    A. In-situ moisture conservation

    They hold rain where it falls.

17. A farm pond is mainly used to:

    1. Store runoff for supplemental irrigation
    2. Recharge deep aquifers only
    3. Grow mangroves
    4. Measure rainfall
    Answer

    A. Store runoff for supplemental irrigation

    A farm pond is an ex-situ storage structure.

18. The water held between field capacity and permanent wilting point is called:

    1. Capillary rise
    2. Available water
    3. Gravitational water
    4. Hygroscopic water
    Answer

    B. Available water

    Plants can use this range of soil water.

19. Compared with bare land, a good forest cover mainly:

    1. Increases soil loss
    2. Increases surface runoff
    3. Removes baseflow
    4. Reduces and delays peak flood flow
    Answer

    D. Reduces and delays peak flood flow

    Litter and roots increase infiltration and slow flow.

20. Clear felling a forest catchment usually causes, at first:

    1. Lower water yield and less erosion
    2. Higher water yield and more erosion
    3. No change in flows
    4. Higher groundwater recharge only
    Answer

    B. Higher water yield and more erosion

    Less transpiration raises yield; bare soil erodes.

21. Fast-growing high-water-use plantations in dry areas can:

    1. Stop all transpiration
    2. Cause no change in soil moisture
    3. Reduce streamflow and recharge
    4. Always raise water tables
    Answer

    C. Reduce streamflow and recharge

    Their transpiration demand can exceed local supply.

22. A circular watershed tends to give:

    1. A sharp, high peak flow
    2. A low, spread-out peak
    3. No peak flow
    4. Only baseflow
    Answer

    A. A sharp, high peak flow

    All parts contribute to the outlet at about the same time.

23. A high drainage density generally indicates:

    1. Slow runoff
    2. Fast runoff and quick response to rain
    3. Very high infiltration
    4. No stream network
    Answer

    B. Fast runoff and quick response to rain

    Many channels carry water out quickly.

24. Riparian vegetation along streams mainly:

    1. Increases bank cutting
    2. Blocks all baseflow
    3. Raises stream temperature
    4. Filters sediment and stabilises banks
    Answer

    D. Filters sediment and stabilises banks

    Root mats hold banks and trap silt.

25. Fog drip is most associated with:

    1. Open grassland
    2. Hot deserts
    3. Cloud forests
    4. Saline flats
    Answer

    C. Cloud forests

    Canopies comb fog droplets, adding water.

26. Reservoir life can be extended by:

    1. Felling trees upstream
    2. Burning catchment cover
    3. Afforestation and soil conservation in the catchment
    4. Ploughing slopes up and down
    Answer

    C. Afforestation and soil conservation in the catchment

    Less erosion means less siltation.

27. The rational formula is best suited to:

    1. Small catchments
    2. Entire river basins
    3. Oceans
    4. Glaciers only
    Answer

    A. Small catchments

    It assumes uniform rain over a small area.

28. The runoff coefficient is generally lowest for:

    1. Rock outcrops
    2. Dense forest
    3. Roof surfaces
    4. Paved roads
    Answer

    B. Dense forest

    Forest soil absorbs most rain.

29. Using Q = C i A / 360 with C = 0.4, i = 90 mm/h and A = 36 ha, Q is:

    1. 1.4 m3/s
    2. 14.4 m3/s
    3. 36 m3/s
    4. 3.6 m3/s
    Answer

    D. 3.6 m3/s

    0.4 x 90 x 36 / 360 = 3.6.

30. Using the rational formula, C = 0.5, i = 60 mm/h, A = 72 ha gives Q equal to:

    1. 3 m3/s
    2. 6 m3/s
    3. 60 m3/s
    4. 12 m3/s
    Answer

    B. 6 m3/s

    0.5 x 60 x 72 / 360 = 6.

31. C = 0.25, i = 120 mm/h and A = 24 ha. Peak discharge is:

    1. 4 m3/s
    2. 20 m3/s
    3. 1 m3/s
    4. 2 m3/s
    Answer

    D. 2 m3/s

    0.25 x 120 x 24 / 360 = 2.

32. Rain of 50 mm falls on 10 ha with runoff coefficient 0.3. Runoff volume is:

    1. 500 m3
    2. 1,000 m3
    3. 1,500 m3
    4. 5,000 m3
    Answer

    C. 1,500 m3

    Rain volume 5,000 m3; 0.3 x 5,000 = 1,500 m3.

33. Rain of 80 mm falls on 5 ha. The total rain volume is:

    1. 800 m3
    2. 400 m3
    3. 40,000 m3
    4. 4,000 m3
    Answer

    D. 4,000 m3

    80 x 5 x 10 = 4,000 m3.

34. Gross rainfall is 40 mm, interception loss 6 mm. Net rainfall reaching the ground is:

    1. 40 mm
    2. 6 mm
    3. 34 mm
    4. 46 mm
    Answer

    C. 34 mm

    Net = 40 - 6 = 34 mm.

35. A watershed has 30 km of streams in 15 sq km. Drainage density is:

    1. 45 km per sq km
    2. 2 km per sq km
    3. 0.5 km per sq km
    4. 15 km per sq km
    Answer

    B. 2 km per sq km

    30 / 15 = 2.

36. Annual rainfall 900 mm, evapotranspiration 600 mm, no change in storage. Runoff is:

    1. 300 mm
    2. 150 mm
    3. 1,500 mm
    4. 500 mm
    Answer

    A. 300 mm

    Q = P - ET = 300 mm.

37. A tank of 2 ha water-spread has average depth 1.5 m. Its volume is:

    1. 3,000 m3
    2. 30,000 m3
    3. 300,000 m3
    4. 15,000 m3
    Answer

    B. 30,000 m3

    2 ha = 20,000 m2; x 1.5 m = 30,000 m3.

38. Consider the statements: 1. Net rainfall equals throughfall plus stemflow. 2. Interception loss reaches the soil as net rainfall. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Interception is evaporated from foliage, so statement 2 is false.

39. Consider the statements: 1. Forests always increase total water yield of a catchment. 2. Forest soil generally has high infiltration. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Forests often use water; infiltration is high under forest.

40. Consider the statements: 1. Percolation tanks are used for groundwater recharge. 2. Contour trenches are ex-situ structures. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Contour trenches conserve moisture in situ.

41. Consider the statements: 1. Kolleru in Andhra Pradesh is a Ramsar wetland. 2. In the rational formula Q is in m3/s when i is in mm/h and A in ha. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are correct.

42. Consider the statements: 1. Infiltration is downward movement of water inside soil. 2. Percolation is entry of water at the soil surface. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    D. Neither 1 nor 2

    The definitions are swapped; both statements are wrong.

43. Consider the statements: 1. Mulching and bunding conserve moisture in situ. 2. A lysimeter measures evapotranspiration. Which of the statements is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

44. A runoff volume of 1,500 m3 comes from a 10 ha catchment. The runoff depth is:

    1. 1.5 mm
    2. 150 mm
    3. 30 mm
    4. 15 mm
    Answer

    D. 15 mm

    1,500 / (10 x 10 m3 per mm per ha) = 15 mm.

45. Organic matter and humus in forest soil mainly:

    1. Increase surface sealing
    2. Raise water-holding capacity and infiltration
    3. Remove available water
    4. Lower infiltration
    Answer

    B. Raise water-holding capacity and infiltration

    Humus improves structure and holds more usable water.