1. Estimating Recharge on Groundwater Resources Projects The Influence of Soils & Crops Tim Hess Institute of Water & Environment Cranfield University Environment Agency Workshop - 4 th November 2004

2. Potential Recharge Recharge gets what’s left …. Precipitation Interception loss Overland flow Soil Evaporation Transpir- ation Storage

3. Interception loss Site % of annual rainfall Source Upland Forest25 - 49Johnson (1990) Lowland Beech14 -16Neal et al. (1993) Short Rot n Coppice21Rushton (1998) Interception loss

4.  With short vegetation, canopy evaporation may (partially) substitute for transpiration Source: Thompson et al., 1996 Interception loss Smaller net ‘loss’

5. Spatial redistribution Estimated rainwater entering lysimeters containing 3 different sized willow plants during 3 rainfall events. Seymour, Pers. Com. Interception loss

6. Soil evaporation  High when wet  Low when dry Time since wetting E/PE 1.0 Wet soil Energy limited Dry soil Supply limited  Low in winter (PE is low)  Important in spring (E soil  ½ ET grass )  Low in summer (soil cover is high) Soil Evaporation

7. ET and plants  Kc = Potential ET / Reference ET (ETo  ETref  “PET”  PE)  Kc = f(plant type, density, height)  Kc varies seasonally Transpir- ation

8. Kc curve for an annual crop  For most agricultural crops, 0.9 < Kc <1.2 at full cover  Cover development is more important  Sources: FAO56, “Crop Calendars” Transpir- ation

9. Plant 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 QsQs Transpir- ation

10. Soil Storage Soil storage  Field Capacity  Permanent Wilting Point  Drainage rate  Rooting depth (season)  Fraction that is “easily” available i.e. when ET = potential  Together, these determine …….  Volume of storage  Rate of depletion through ET  Rate of depletion through drainage

11. Issues - interception  How much precipitation never reaches the ground?  How does this vary seasonally?  When is it important and when can we ignore it?

12. Issues - 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 Kc ini is not suitable for rainfed conditions when Kc = f(wetting)  Is interception accounted for in the Kc? Transpir- ation

13. Issues – ET continued  What is the Kc to use for non-agricultural surfaces?  ‘Natural’ vegetation  Wetlands  What about ‘edge effects’, stand size and advection? Transpir- ation

14. Issues - 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?

15. … and finally ….  Recharge = precipitation - interception loss - evaporation - surface runoff (+ runon) - actual transpiration  +/- errors in all of the above!