Crop Water Balance

Colin S. Campbell, Ph.D.

Decagon Devices

Groundwater

Water Balance

Water in – water out = water storedWater in

Precipitation (irrigation)Water out

Infiltration (deep drainage)Evapotranspiration Runoff

Water storedSoil water content

Precip – deep drainage – ET – Runoff = water storage in root zone

Soil Water Cycle(100 cm precip. with vegetative cover)

Precipitation, dewfall(or irrigation)

runoff

transpiration

evaporation

Groundwater

DrainageStorage

(soil moisture)

100 cm

40 cm

10 cm

15 cm

35 cm

Upper boundary Precipitation/Irrigation

Precipitationrainfall, snowfall

IrrigationSprinkler, drip, etc.

Upper boundary Runoff

A significant portion of precipitation on the soil surface may not be absorbed

Dependent on precipitation/snowmelt intensity

TopographyGround cover/vegetation

Upper boundary Evapotranspiration (ET)

ET Covers all water loss to the atmosphere

Evaporation (E)Direct soil to atmosphere transferDominant 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 atmosphereVapor diffusion slow

Plant canopy decreases E dramatically

Transpiration (T)Occurs via plantsDominates over moderately and densely

vegetated surfaces (shade soil from radiation)

Draws water from deeper in the soil profile

Unaffected by dry surface layer

Upper boundary Evapotranspiration

Lower BoundaryDrainage

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

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

Measurement Upper boundary

RunoffCatchment flow (measure stream

flow)– ecosystem scale studiesRunoff collectors – measure runoff

from specific slope/locationHigh flow tipping bucket flow meter

Measurement Upper boundary

EvapotranspirationSap Flow

Good estimate of transpirationTemperature rise at heated needle

inversely proportional to sap flowMust scale up from individual stems

(trunks) to full ecosystem scale

See Wilson et al., 2001 for measurement comparison

Measurement Upper boundary

EvapotranspirationBiophysical modeling

Use measurable environmental parameters to calculate evapotranspiration

Penman – Monteith, Priestley – Taylor modelsCalculate reference evapotranspirationApply crop coefficient for particular canopy

Models often require many measurements solar radiation, wind speed, air temp, soil temp, etc

Error terms can be large

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.

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 storedMore on sensor types in practicum

Measurement Moisture storage in soil

Example If average sensor readings change from 0.150 to

0.160 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

0.005 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

MeasurementLower Boundary - drainage

Water balance residual methodPrecipitation + 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

Precipitation, dewfall(or irrigation)

runoff

transpiration

evaporation

Groundwater

DrainageStorage

(soil moisture)

Soil Hydrologic Cycle(150 cm precip. with vegetative cover)

150 cm60 cm

15 cm

30 cm

45 cm

10% error in ET = 17% error in Drainage

Precipitation, dewfall(or irrigation)

runoff

transpiration

evaporation

Groundwater

DrainageStorage

(soil moisture)

Soil Hydrologic Cycle(20 cm precip. with little vegetative cover)

20 cm

4 cm

11 cm

4.5 cm

0.5 cm

10% error in ET = 300% error in Drainage

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

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

Controlled tension and weighing lysimeters

Major installation effort Accurate and precise Expensive

ExampleOrange 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

0

4

8

12

16

20

8/1 8/4 8/7 8/10 8/13 8/16 8/19 8/22 8/25 8/28 8/31

August 2006

Vo

lum

etri

c w

ater

Co

nte

nt

(%)

0

4

8

12

16

20

Rai

nfa

ll (

mm

)

EC-5 15cm EC-5 30cm EC-5 45cm

EC-5 90cm TE-5(WC) 15cm Rain (mm) 0

Data courtesy of W. Bandaranayake and L. Parsons, Univ. of Florida Citrus Research and Education Center

ExampleSoil volumetric water content

ExampleOrange grove water balance

Change in VWC = 0.070 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

residual11.7 mm – 7 mm = 4.7 mm estimated

leaching to ground water

ExampleEvaluation: 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

Average Precipitation or irrigation

runoff

evapotranspiration

Groundwater

DrainageStorage

(soil moisture)

Average Daily Soil Water CycleOrange Grove

11.4 mm

7 mm

0 mm

4.7 mm

References

Baldocchi, D. D., B.B. Hicks, and T.P. Meyers. 1988. Measuring biosphere-atmosphere exchanges of biologically related gases with micrometeorological methods. Ecology 69: 1331-1340

Methods of Soil Analysis Part 4: Physical Methods. 2002. J.H. Dane and G.C. Topp eds. SSSA Book Series 5, Madison, WI. Chapter 3, Soil Solution Phase.

Rutter, A.J., 1975. 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. 2001. 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): 153-168.