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

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Presentation on theme: "Crop Water Balance Colin S. Campbell, Ph.D. Decagon Devices Groundwater."— Presentation transcript:

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

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

4 Upper boundary Precipitation/Irrigation Precipitation 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 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 Upper boundary Evapotranspiration

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 m 3 /m 3 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 m 3 /m 3 is equivalent to a m 3 water volume increase in a soil volume with 1 m 2 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 (or irrigation) runoff transpiration evaporation Groundwater Drainage Storage (soil moisture) Soil Hydrologic Cycle (150 cm precip. with vegetative cover) 150 cm 60 cm 15 cm 30 cm 45 cm 10% error in ET = 17% error in Drainage

18 Precipitation, dewfall (or irrigation) runoff transpiration evaporation Groundwater Drainage Storage (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

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

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 ECH 2 O EC-5 probes buried through root zone Find: Water leaching to ground water using residual technique

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

24 Example Orange grove water balance Change in VWC = m 3 / m 3 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 runoff evapotranspiration Groundwater Drainage Storage (soil moisture) Average Daily Soil Water Cycle Orange Grove 11.4 mm 7 mm 0 mm 4.7 mm

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):


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