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!