Irrigation Scheduling and Soil Moisture Monitoring in the Vineyard R. Troy Peters, Ph.D., P.E. Extension Irrigation Engineer Washington State University, IAREC Prosser, WA
Soil Water Saturation Field Capacity (FC) Excess Water Permanent Wilting Point (PWP) Available Soil Moisture Total pore space Unavailable Water Oven Dry
110% 100% 90% Production Reduction Function % of Maximum Pro oduction 80% 70% 60% 50% 40% 30% 20% FC MAD PWP 10% 0% 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% % of Available Water
Soil Texture and Available Water Soil Texture Available Water (AW) in/ft Coarse Sand 0.2-0.8 Fine Sand 0.7-1.0 Loamy Sandy 0.8-1.3 Sandy Loam 1.1-1.6 Fine Sandy Loam 1.2-2.0 Silt Loam 1.8-2.8 Silty Clay Loam 1.6-1.9 Silty Clay 1.5-2.0 Clay 1.3-1.8 Peat Mucks 1.9-2.9
Example on Sandy Soil Fine Sand: AWC = 0.7 in/ft Effective rooting depth: 4 ft Total water holding capacity: 0.7 in / ft x 4 ft = 2.8 in MAD: 50% Soil water deficit at MAD: 2.8 in * 50% = 1.4 in (maximum depletion)
Example on Silt Loam Soil Sl Silt Loam: AW = 2 in/ft Effective rooting depth: 4 ft Total water holding capacity: 2 in / ft x 4 ft = 8 in MAD: 50% Soil water deficit at MAD: 8 in * 50% = 4 in (maximum depletion)
Managing to Minimize Stress 20 Field Capacity 15 MAD inche es of water 10 5 Wilting Point Irrigation + Rain 0 Neutron Probe Reading Deep Percolation -5 104 112 120 128 136 144 152 160 168 176 184 192 200 208 216 224 232 240 248 256 264 272 280 day of year
Poor Irrigation Scheduling 20 FC 15 MAD inch hes of water 10 5 WP Irrigation + Rain 0 Neutron Probe Reading Deep Percolation -5 104 112 120 128 136 144 152 160 168 176 184 192 200 208 216 224 232 240 248 256 264 272 280 day of year
Regulated Deficit Irrigation
Washington Ag Weather Network Speed- ometer http://weather.wsu.edu/
Kansched Google it! MS Excel
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Soil Moisture Sensors (gas gauge)
Where to put them? Field location Not right on the edge of the field. Average soil condition? Or, lowest water holding capacity (sandy). Depths (1) Middle of root zone. (2) 1/3 middle, and bottom of root zone. (3) Top, middle, and bottom of root zone. Top sensors tell you when to water, bottom shows more severe stress, and shows water holding capacity of soil how much.
Neutron Probe
Strengths Neutron Probe Accurate Gives soil water content Large soil sample area Unaffected by salinity or temperature Repeatable Easy to sample at different depths Weaknesses Highly regulated (nuclear device) Can t leave in the field Expensive
Capacitance Probes
Capacitance ce Sensors s Strengths Gives soil water content Easy to log data (real-time) Weaknesses Small sample area Highly dependant on very local soil structure Inconsistent (high variability) Can be expensive Proper installation ti is critical, and not always easy Affected by salinity and temperature
Practical use of soil content sensors (capacitance, neutron probe) Measure as soon as soil thaws = field capacity, or 24 hrs after heavy irrigation Then use AWC by soil texture and start with 50% MAD for grapes. Refine this using feedback from plants and experience.
Tensiometers
Tensiometers Strengths Soil water tension (same as plant sees) Less expensive Widely used, studied d and accepted Not affected by salinity Weaknesses Small sample area Indicates when to irrigate, not how much
Resistance
Resistance type Strengths Inexpensive Usable trends Give soil water potential ti (same as plant sees) Easy to log data Weaknesses Affected by salinity Imperfect accuracy Samples small area
Practical use of Tension-Based Soil Moisture Sensors Start with 40-60 centibars as irrigation point for no stress. Higher for imposed stress. Correlate numbers to crop condition, then use numbers in future to indicate degree of soil dryness. Watch deep sensor after an irrigation to indicate depth of penetration.
Soil Moisture Sensors The Look and Feel Method Advantagesd Cheap Easy Weaknesses Subjective - especially w/ dry soil.
Look and Feel May take time and experience to train yourself to do this correctly. Start with the NRCS pamphlet recommendations Refine your techniques for your particular soil by condition of soil at particular crop stress points, or correlated with a soil sensor. Go deep!
Soil Moisture Sensors Neutron Probe will give the most correct answer. Most sensors will give a trend that is usable for irrigation scheduling. Proper installation of sensors is critical and must be done right for good data Actually using the sensor is most important! Keep records calibrate yourself using numbers, crop response and experience Hire a consultant and watch them.
$4,500 Most important thing is: Do Something! Stop Guessing! You can t manage what you don t measure. $50
Water is still the best fertilizer R. Troy Peters WSU Extension Irrigation Engineer