estimating recharge on groundwater resources projects the influence of soils & crops tim hess...
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Estimating Recharge on Estimating Recharge on Groundwater Resources ProjectsGroundwater Resources Projects
The Influence of Soils & Crops
Tim Hess
Institute of Water & Environment
Cranfield University
Environment Agency Workshop - 4th November 2004
Potential Recharge
Recharge gets what’s left ….Recharge gets what’s left ….Precipitation
Interception loss
Overland flow
Soil Evaporatio
n
Transpir-ation
Storage
Interception lossInterception loss
Site % of annual
rainfall
Source
Upland Forest 25 - 49 Johnson (1990)
Lowland Beech 14 -16 Neal et al. (1993)
Short Rotn Coppice 21 Rushton (1998)
Interception loss
Interception lossInterception loss
With short vegetation,
canopy evaporation may
(partially) substitute for
transpiration
0
1
2
3
4
5
6
7
8
9
10
Sprinkler Surface
Irrigation method
mm
Transpiration Canopy Evaporation Soil Evaporation
Source: Thompson et al., 1996
Interception loss
Smaller net ‘loss’
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
1-Oct-01 8-Oct-01 9-Oct-01
Date
Rai
n w
ater
(m
l)
Rainfall Small Medium Large
Spatial redistributionSpatial redistribution
Estimated rainwater entering lysimeters containing 3 different sized willow plants during 3 rainfall events. Seymour, Pers. Com.
Interception loss
Soil evaporationSoil evaporation
High when wet
Low when dry
Time since wetting
E/P
E
1.0Wet soilEnergy limited
Dry soilSupply limited
Low in winter (PE is low)
Important in spring (Esoil ½ ETgrass)
Low in summer (soil cover is high)
Soil Evaporatio
n
ET and plantsET and plants
Kc = Potential ET / Reference ET(ETo ETref “PET” PE)
Kc = f(plant type, density, height)
Kc varies seasonally
Transpir-ation
Kc curve for an annual cropKc curve for an annual crop
0.0
0.2
0.4
0.6
0.8
1.0
1.2
01-Apr 01-May 01-Jun 01-Jul 01-Aug 01-Sep 01-Oct
Kc
20 35 40 30
0.35 1.14
0.60
For most agricultural crops, 0.9 < Kc <1.2 at full cover
Cover development is more important
Sources: FAO56, “Crop Calendars”
Transpir-ation
Plant stressPlant stress
Qs < ET = stress stomata close
ET < potential
Occurs when ET is high (midday, summer)
Qs is low (dry soil, sandy soil)
ETactual depends on Weather
Soil water content
ET
Qs
Transpir-ation
Soil Storage
Soil storageSoil storage
Field Capacity
Permanent Wilting Point
Drainage rate
Rooting depth (season)
Fraction that is “easily” availablei.e. when ET = potential
Together, these determine ……. Volume of storage
Rate of depletion through ET
Rate of depletion through drainage
Issues - interceptionIssues - interception
How much precipitation never reaches the
ground?
How does this vary seasonally?
When is it important and when can we
ignore it?
Issues - ETIssues - ET
How is Kc defined?
What is the reference used (Penman,
Penman-Monteith, MORECS, MOSES)?
Which was used in the development of Kc?
FAO 56 approach to Kcini is not suitable for
rainfed conditions when Kc = f(wetting)
Is interception accounted for in the Kc?
Transpir-ation
Issues – ET Issues – ET continuedcontinued
What is the Kc to use for non-agricultural
surfaces?
‘Natural’ vegetation
Wetlands
What about ‘edge effects’, stand size and
advection?
Transpir-ation
Issues - soilIssues - soil
What fraction of total available water is “easily”
available?
FAO56 “p” factor developed for monthly water
balances
How well do the models account for wetting on dry soils?
slow release of drainage?
How important is small-scale spatial variability in
soil, plants and wetting patterns?
… … and finally ….and finally ….
Recharge = precipitation
- interception loss
- evaporation
- surface runoff (+ runon)
- actual transpiration
+/- errors in all of the above!