overland flow
water reaching stream while traveling above ground
Subsurface flow
water reaching stream through shallow soil & underlying rock layer
-Shallow subsurface flow SSSF = upper soil layer
-GW flow (GWF) = deeper soil & rock layer
HOF: Infiltration-excess overland flow
occurs when rainfall rate exceeds soil infiltration rate (more likely in clay soil = slower f(t))
rainfall excess
rainfall neither retained on land surface nor infiltrated into soil
Excess rainfall = observed rainfall- hydrologic abstraction
HOF occurence
road surface & impermeable areas
hydrophobic soils
trampled & crusted soils
Frozen ground
Low-permeability soils
SOF : saturation-excess overland flow
when soil is fully saturated and no space for additional water infiltration, any additional precipitation becomes SOF
-mainly occurs on topographic low & near streams (water table reaching surface)
Return flow
Subsurface water pushed towards surface. soil water properties affect water quality of return flow.
Variable source area
expanding/shrinking saturated area during/after rainfall event
Assessing landscape overland flow
consider seasonality AND storm fluctuation
more humid with less rainfall intensity = SOF
-more arid with high rainfall intensity = HOF
matrix flow
water moving through small soil pores
subsurface fill and spill mechanism
water spill from one depression to another at soil-bedrock interface
macropore flow
water moving through larger soil pores
pipeflow
water moving through large soil cavities, bypassing soil matrix.
-frequent in organic soils
GW flow
water move below water table in soil/ rock
types of subsurface flow
matrix flow
fill and spill mechanism
pipeflow
macropore flow
GW flow
Shallow subsurface stormflow development
- at start, percolation occurs vertically
- soil moisture increase, some water bypassing to depth
- water table rise towards soil surface
- before water table reach surface, hydraulic condition change -> Lateral hydraulic conductivity > vertical hydraulic conductivity
perched SSSF development
1.Same as SSSF but, percolation reaches impeding layer and saturation occurs.
2. perched water table rise and before reaching surface hydraulic conductivity changes (lateral>vertical)
subsurface fill & spill development
impermeable boundaries restricting water percolation causes bedrock hollow to fill and spill -> forming continuous flow
Macropore flow characteristics
-depends on pore size distribution & interconnection of pores overtime, forming a network
-lots of macropores are biopores
-preferential flow in macropores
-permeable walls -> when saturated, lateral infiltration occur via macropore flow
preferential runoff pathway definition
uneven and often rapid movement of water & solutes through soil at a faster than normal matrix flow, so that a small fraction of the media participates in most of the flow
preferential flow occurence
-distinctive structure of soil where water flows only under gravity
-areas with higher permeability than surrounding soil matrix
- occurs with: Macropore, pipe and matrix flow (perched SSSF, fill & spill)
impact of runoff on water quality
-influence base cation/anion concentration as water pick up ions by traveling through soil
-large effect on nutrient & sediment flushing and reaching streams
spatiotemporal variability of runoff flows
-can have multiple overland & subsurface flow types at same time
-antecedent wetness conditions are IMP
role of watershed characteristics in runoff flow
forest: high evapotranspi, low surface runoff, higher subsurface flow/infiltration
Degraded land: lower evapotranspi, higher surface runoff, medium subsurface flow/infiltration
