1. Colin S. Campbell, Ph.D. Decagon Devices
Crop Water Balance
Colin S. Campbell, Ph.D.
Decagon Devices
Groundwater

2. Water in – water out = water stored
Water Balance
Water in – water out = water stored
Water in
Precipitation (irrigation)
Water out
Infiltration (deep drainage)
Evapotranspiration
Runoff
Water stored
Soil water content
Precip – deep drainage – ET – Runoff = water storage in root zone

3. Precipitation, dewfall
Soil Water Cycle
(100 cm precip. with vegetative cover)
transpiration
40 cm
evaporation
100 cm
Precipitation, dewfall
(or irrigation)
10 cm
runoff
15 cm
Drainage
Storage
(soil moisture)
35 cm
Groundwater

4. Upper boundary Precipitation/Irrigation
rainfall, snowfall
Irrigation
Sprinkler, drip, etc.

5. Upper boundary Runoff
A significant portion of precipitation on the soil surface may not be absorbed
Dependent on precipitation/snowmelt intensity
Topography
Ground cover/vegetation

6. Upper boundary Evapotranspiration (ET)
ET Covers all water loss to the atmosphere
Evaporation (E)
Direct soil to atmosphere transfer
Dominant process over bare soil or sparse vegetation (energy available for evaporation)
Severely retarded by a layer of dry soil or mulch between wet soil and atmosphere
Vapor diffusion slow
Plant canopy decreases E dramatically

7. Upper boundary Evapotranspiration
Transpiration (T)
Occurs via plants
Dominates over moderately and densely vegetated surfaces (shade soil from radiation)
Draws water from deeper in the soil profile
Unaffected by dry surface layer

8. Lower Boundary Drainage
Drainage - water that percolates past root zone
Very little brought back up by capillary rise
Most percolates down into groundwater
Traditionally difficult to measure
Very important parameter for
Water balance
Aquifer recharge
Groundwater contamination

9. Measurement Upper boundary
Precipitation
Variety of gauges available for different price and accuracy/resolution needs
Not as easy to measure as many people think
Irrigation
Above ground (sprinkler, etc)
Rain gauge will work for both precipitation and irrigation
Below ground (drip, etc.)
In some cases, it is possible to measure application water with a flow meter
Other cases require reliance on water content monitoring

10. Measurement Upper boundary
Runoff
Catchment flow (measure stream flow)– ecosystem scale studies
Runoff collectors – measure runoff from specific slope/location
High flow tipping bucket flow meter

11. Measurement Upper boundary
Evapotranspiration
Sap Flow
Good estimate of transpiration
Temperature rise at heated needle inversely proportional to sap flow
Must scale up from individual stems (trunks) to full ecosystem scale
See Wilson et al., 2001 for measurement comparison

12. Measurement Upper boundary
Evapotranspiration
Biophysical modeling
Use measurable environmental parameters to calculate evapotranspiration
Penman – Monteith, Priestley – Taylor models
Calculate reference evapotranspiration
Apply crop coefficient for particular canopy
Models often require many measurements solar radiation, wind speed, air temp, soil temp, etc
Error terms can be large

13. Measurement Upper boundary
Evapotranspiration –
Micrometeorological measurements (see Baldocchi et al., 1988)
Direct measurement of surface-atmosphere exchange of gases (water vapor)
Eddy Covariance, Bowen ratio, Flux gradient, Conditional sampling, etc.

14. Measurement Moisture storage in soil
Log change in volumetric water content (VWC) over time in the root zone
Multiple VWC measurements throughout the soil profile give you amount of water stored
More on sensor types in practicum

15. Measurement Moisture storage in soil
Example
If average sensor readings change from to m3/m3 over a depth of 0.5 m, how much water infiltrated into the soil?
Assume probes spaced evenly over 0.5 m depth
A change in VWC of 0.01 m3/m3 is equivalent to a m3 water volume increase in a soil volume with 1 m2 ground area and 0.5 m depth.
This suggests 5 mm (0.005m) of water infiltrated into the soil

16. Measurement Lower Boundary - drainage
Water balance residual method
Precipitation + Irrigation = Runoff +
Storage + Evapotranspiration + Drainage
Measure precipitation & irrigation
Measure or estimate runoff
Measure or estimate ET
Measure soil water storage (volumetric water content)
Drainage is calculated as whatever is left over

17. Precipitation, dewfall
Soil Hydrologic Cycle
(150 cm precip. with vegetative cover)
10% error in ET =
17% error in Drainage
transpiration
60 cm
Precipitation, dewfall
(or irrigation)
150 cm
evaporation
15 cm
runoff
30 cm
Drainage
Storage
(soil moisture)
45 cm
Groundwater

18. Precipitation, dewfall
Soil Hydrologic Cycle
(20 cm precip. with little vegetative cover)
10% error in ET =
300% error in Drainage
transpiration
4 cm
evaporation
20 cm
Precipitation, dewfall
(or irrigation)
11 cm
runoff
4.5 cm
Drainage
Storage
(soil moisture)
0.5 cm
Groundwater

19. Zero tension (pan) lysimeters
Most basic measurement of drainage - simple collection pan buried in soil
Serious problems with flow divergence
Collection efficiencies of < 10% are common
Common in landfill liner applications

20. Wick Lysimeters
Wick (hanging water column) used to pull tension on soil water
Static tension chosen to optimize water collection efficiency
Good accuracy in most soils
Mid level performance, mid level price

21. Controlled tension and weighing lysimeters
Major installation effort
Accurate and precise
Expensive

22. Example Orange grove water balance
Site background
Orange grove grown in 97% sand soil
Precipitation measured by rain gauge but irrigation is unknown
Local meteorological data available for ET calculation
ECH2O EC-5 probes buried through root zone
Find: Water leaching to ground water using residual technique

23. Example Soil volumetric water content
Data courtesy of W. Bandaranayake and L. Parsons, Univ. of Florida Citrus Research and Education Center

24. Example Orange grove water balance
Change in VWC = m3/ m3 over 6 days
Probes in top meter of soil so 11.7 mm of water lost out of the root zone per day
If ET is estimated as 7 mm per day
Estimate water leaching to ground water
Runoff can be ignored in the sand
Deep percolation is calculated as the residual
11.7 mm – 7 mm = 4.7 mm estimated leaching to ground water

25. Example Evaluation: How to improve estimate
The numbers that we calculated are a VERY rough estimate
Error in wet climate may be very low because overall water flux is large while error in dry climates can be very high
We could improve these numbers by:
Measuring deep drainage with a lysimeter
Measuring ET by
Sap flow in the orange tree to get actual Transpiration
Or calculate ET using Pennman-Montieth and crop coefficient

26. Average Precipitation or irrigation
Average Daily Soil Water Cycle
Orange Grove
Average Precipitation or irrigation
evapotranspiration
11.4 mm
7 mm
runoff
0 mm
Drainage
Storage
(soil moisture)
4.7 mm
Groundwater

27. References
Baldocchi, D. D., B.B. Hicks, and T.P. Meyers Measuring biosphere-atmosphere exchanges
of biologically related gases with micrometeorological methods. Ecology 69:
Methods of Soil Analysis Part 4: Physical Methods J.H. Dane and G.C. Topp eds. SSSA Book
Series 5, Madison, WI. Chapter 3, Soil Solution Phase.
Rutter, A.J., The Hydrological Cycle in Vegetation. In Vegetation and the Atmosphere.
Volume I: Principles J.L. Monteith Ed. Academic Press, New York.
Wilson, K.B., P.J. Mullholland, D.D. Baldocchi, others A comparison of methods for
determining forest evapotranspiration and its components: sap-flow, soil water
budget, eddy covariance, and catchment water balance. Agricultural and Forest
Meteorology 106 (2):