Water Cycles and Management
Run-off Factors Depend On:
• Absolute catchment size: “small” catchments (stream orders 1-3) discharge a greater proportion of water and have greater flood flow than “large” catchment (stream orders 4-6). E.g. foothills will peak to flood more rapidly than plains, and may discharge 30-40% of rain. On plains only 8- 20% may occur as discharge.
• Stream gradient: Also exacerbates rapid discharge.
• Absolute storm size: Large storms may wet all of a small catchment without causing much run-off in lower catchment.
• Retardation swamps and basins: Swales, dams, swamps, basins, water spreading generally delay or obviate flood and run-off. These are susceptible to our management. Sand beds and sand sheets have similar effects.
• Catchment form: Dendritic (tree form) catchment the most efficient. Thus fold “trellis” and fault catchments discharge less, more slowly.
• Drainage density: Dense streamlines remove water quickly; interfluves retard flow, as do braided streams.
• Vegetation: Type and density affect run-off. Trees and tussocks retard; ephemerals and annuals or bare areas increase run-off, as does burning or cultivation.
• Sealed and rock areas: increase run-off. Also map soils and rock types, shattered “intake” areas, boulder fields, sand cover.
• Water viscosity: (temperature) has both diurnal and seasonal effects.
• Rain duration: a primary factor. Roughly a 4-5 year “wet” needed to flood basins, 2-3 years to recede.
Water Control: Managing Surface Run-off in Broad Landscape
General rules:
• Modest trials; extend on successes
• Do not concentrate flow across even low slopes or they will gully out
• Always spread and absorb flow in pits, swales, sands.
• Try to totally absorb runoff into vegetated areas
• Beware absorbing runoff into bare areas as this can raise the water table and cause salting
• Plant trees over shallow water tables
• Water flow-wadi)
Arroyos:
• Rock-cut channels on scarp, back slope and in pediment rock will lead water to walled fields (walls 1.5m high). Water gates allow each field to be flooded or to spill to the next section for field crop.
• Small arroyos can be completely dammed across and sand absorption practiced.
• Large arroyos need central floodway left open to spill to large, low-walled fields in plains if excess
water falls, these may fill every 6-10 years; not normally planted.
• Shallow flood streams in foothills can be “bled off” at bands to long swales 1-2m deep; overshadowed with trees. Swale base will carry crop seasonally.
Sandy Rivers:
• Can be rock-dammed every 1-2km, at 1-2m deep, and sand allowed to fill dam area; spread floodwaters; absorb.
• Walls can “leak” to next dam; areas can be tree-planted or sub-surface drains led to pipes or a small open rockhole or clearwater dam for aquaculture (this dam: 1/25th of sand dam area or less)
• A notched concrete sill in sandy river beds will spread braided streams across entire flood plain; trees do well along braided channels. Absorption of water commonly 10-20m from streambeds in sands (this may be due to tree root reach).
• A 2-3m high bank towards a stream causes scour pool in the stream bed (1/100th of area of streambed will be permanent).
• Low permanent walls in flood plains retain silt and water for crop.
Slopes Generally:
• Swales at 1-3m vertical intervals, swales of 2m deep, broad (3-5m); tree planted.
• Steeper slopes can be sculptured as “net and pan”, “boomerang” banks.
• Low slopes (3°) set out as a series of large diamond-shaped basins of 0.5 hectare, each with a tree in the low corner, and a spillway to the next diamond; even light rain soaks into these. Try for 150-200m deep ponds in small areas.
Plains:
• Dams between hills by a 4-cast bulldozer bank 1-5km long, banks 1-2m high, backing up water for 2-10km. This is then released over next 4-8 days into chiseled fields 1/20th the size of the flooded area, and allowed to soak in by a succession of floodings. Crops are planted in both the dammed area and the
fields. A bypass canal allows spillage of excess water.
• Patterned with great circles (30-50m across) cast out by blades, squares, etc. to totally prevent run-off, and pelleted seed sown to grow rangeland plants.
Dunes:
• Bases are good tree sites or whole dunes can be sown to oats in wet periods, with pelleted Acacia seed to stabilize for permanent effects. Fertilizer (ammonia, zinc, iron, trace elements, molybdenum) is applied often and in small quantities- ideally in pelleted seed.
• Between longitudinal dunes, low diversion banks will deflect run-off into dune sands and trees; repeat every .05-1.5km or more frequently on broad pavements; plant trees in sands. Try for ratio of 20:1 run-off: absorption.
• Fences 1m high, 7m apart, 40% penetrable stabilize dunes; need Acacia or other hardy trees to make this permanent. Trees need a “collar” and pit 15cm or more deep to reduce heat on roots.
Claypans:
• Spiral or ridge up to hold water, sow parallel seed, pit seed and spray on bituminous emulsion to prevent bird losses.
• Allow only very light browsing until shrubs establish. Banks will later “melt” but can be remade in
2-3 sequences until area is re-vegetated.
Extensive:
• “Pitting” for large, sandy areas. Series of pits or mini-swales 15-30cm deep- stagger across slopes.
• Seed and fertilize (pellet) area. Rains will soak into pits, establish plant cover and later shrubs.
• Totally prevents dust loss, dust storms from area. Important around towns, airports, downwind areas of desert. Permanent only if carefully stocked and if trees are later established from pelleted seed.
Gibber or reg:
• Sidecast pebbles to contoured swales to create tree sites in pebble lines. Test plantings before extensive
swales are formed as area can blow to dust where pebbles are shifted. Pebble swales are good tree sites.
Types of Channels
• Swales
• Interceptor banks
• Spreader banks
• Diversion banks
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