crop water balance colin s. campbell, ph.d. decagon devices groundwater
TRANSCRIPT
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.