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3.1 Drainage Basin Water Balance (Input: Precipitation (Stores (Ground…

- - - - Channel Storage
        
        Receives inputs directly from other flows or stores and from precipitation itself
      - Surface Storage
        
        As water reaches the surface, ponding occurs and forms depression storage due to the local irregularities and pits on the surface
        
        Continued rain causes overflowing of the storage and overland flow
        
        Occurs on the surface in small depressions or large storages (lakes,reservoirs,canals)
    - - Interception Storage :evergreen_tree:
        
        In Humid Tropics
        
        Interception: The process where precipitation is caught by vegetation and stored on the canopy
        
        Of the water intercepted, some are evaporated(interception loss) before reaching the surface
        
        A larger amount falls on the ground through leaf-drip (flow of raindrops from leaf tips), stemflow (flow of water down stems and trunks to reach the soil) and throughfall (flow of water through the leaves/caught between)
    - - Upper Soils
        
        Soil Moisture Storage
        
        Refers to the water that is stored within the soil pores after water infiltrates into the soil, while the rest moves ↓ due to gravity
        
        Porosity determines the max amt of water that can be stored in the soil, when the soil is saturated
        
        Storage is only cursory as infiltrated water will percolate to subsurface soil and rock layers to be stored as groundwater
        
        Pore sizes can influence permeability which is how easily water can pass through soil
        
        Pores
        
        Micropores: <0.1mm
        
        Maropores (root systems): >0.1mm
        
        Pipes (cavities): >1mm and continuous in length
      - Soil & Rock Layers
        
        GW Storage
        
        GW: subsurface water within the phreatic zone in highly permeable rock known as aquifers
        
        Recharged when water seeps from vadose zone
        
        WT height affected by frequency of recharge where during prolonged periods of wet weather, WT rises vice versa
        
        GW eventually discharged into river channels as baseflow to seas or oceans
        
        Arid and humid GW thickness differs, arid GW storage varies over short distances
        
        Can accumulate to become a large portion in a year
  - - - Concentrated Flows
        
        Rills & Gullies
      - Generated when ppt at the surface cannot infiltrate or when soils are saturated (infil capacity < rate)
        
        LARGEST pathway
        
        Not considered as part of WB
      - Unconcentrated Sheet Flow
        
        Saturation Overland Flow :evergreen_tree:
        
        Occurs when infiltration capacity is higher than precipitation intensity, when water table is close to the surface and easily saturated
        
        More likely to happen under natural conditions than IEF, in HUMID regions where there are prolonged periods of heavy rainfall
        
        Prolonged rain raises the soil moisture slowly and over time, soils at the base of the slope become saturated and is gradually extended upslope, raising the WT height*
        
        More rain cannot infiltrate and runs over the ground as direct ppt
        
        At the lower parts of the slope, throughflow emerges from the ground and runs down as return flow
        
        SOF = direct ppt onto saturated areas + return flow from throughflow
        
        Position influenced by geology, topography, soils, rainfall characteristics and presence of :evergreen_tree:
        
        Infiltration Excess Flow (Horton Overland Flow) :hot_pepper:
        
        Occurs when rainfall intensity exceeds instantaneous infiltration capacity of an unsaturated soil
        
        Infiltration occurs at capacity rate and water would accumulate on the ground surface
        
        Rare under natural conditions but likely to occur in ARID where rainfall intensity is often high and the soil surface crust is thin and has little vegetation cover or in HUMID during monsoons or heavy rains
        
        Likely to occur in urban areas with impermeable surfaces or in agricultural areas where upper soil layers are compacted by machines or animal grazing
        
        In these cases, infil capacity is limited and overland flow would begin BEFORE storages are even full
        
        Water which cannot infiltrate collects as depression storages and continuous rain would cause them to be filled, flowing as trickles, rivulets or thin sheets of OF
        
        Volume ↑ downslope as water accumulates
        
        IEF occurs at spatially localised parts on a hillslope (arable land), suggesting that parts of the hillslope contributes to IEF rather than the whole slope
    - - Upper Soils
        
        Throughflow
        
        Under gravity, water moves vertically downwards but is affected by flow resistance ↑ as the diameter of pores ↓
        
        Downward flow of water is likely to slow down as permeability ↓ with increasing depth, as soil layers are more compact at the lower layers due to the weight of the soil above
        
        When vertical movement is impeded, water is forced to move laterally in a downslope direction forming throughflow above the water table and concentrated along percolines (line btwn 2 soil horizons) /soil pipes
        
        Common in permeable A horizon
        
        Usually slow flows as mostly matrix flows (small pores) but faster than groundwater flow
        
        Eventually emerges as return flow,
        
        Matrix Flow: micropores, flow of water is slow
        Macropore Flow: rapid flow of water due to the pore size and continuous connectedness over long distances, formed by plant roots (humid)
        Pipeflow: larger versions of macropores like animal burrowing
        
        Common in ARID REGIONS where dessication cracking occurs in drylands and mass movements enlarge the macropores into pipe networks
        
        High hydraulic conductivity (decreases with depth as soil more compact below, water less able to pass)
        
        43% of water moves through pipe networks in semi-arid loess soils of China
        
        Infiltration
        
        As precipitation falls on the slope, water can enter the soil as it moves further underground via other means like stored as soil moisture, throughflow, percolation or groundwater flow and storage
        
        Infiltration: Process by which water is drawn into the soil by the forces of gravity and capillary action
        
        Involves three independent forces, entry into the soil, storage within the soil and transmission through the soil
        
        High infiltration rate due to dry soil but decreases over time as soil might get saturated, blocked by sediment from rainsplash and present clays might swell and reduce pore size
      - Soil & Rock Layers
        
        Base Flow / GW Flow :evergreen_tree:
        
        Rate of discharge in a channel where only flow from aquifers (GW) contributes to channel flow
        
        WT must rise above the channel for this to occur
        
        Constant / Perennial and increases slightly after a wet season
        
        Not common in :hot_pepper:
        
        Percolation :evergreen_tree:
        
        Movement of water downward under the influence of gravity and usually continues downwards to groundwater zone
        
        Rare in arid regions as saturation is rarely reached in soils
        
        Water first passes through the zone of aeration/vadose zone which consists of open pores in the ground filled with air
        
        Then it slowly moves down to enter the groundwater zone and responses to gravity and pressure differences to percolate from areas where WT is high (pressure too) to areas of low WT
        
        zone of saturation/ phreatic zone where all pores are filled with water
        
        The upper surface of the phreatic zone is the water table which follows surface topography and fluctuates with seasonal rainfall
        
        Humid WTs imitate the land above it and arid is expressed as a flat plain
        
        Rate of movement influenced by rock permeability
        
        Can occur long after a storm (longer timeline)