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Soil Management Handbook - Ministry of Agriculture and Lands

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Table 12<br />

Availability Coefficients<br />

Crop Coefficient<br />

Peas 0.35<br />

Potatoes 0.35<br />

Tree Fruits 0.40<br />

Grapes 0.40<br />

Tomatoes 0.40<br />

All other crops until other<br />

data available<br />

0.50<br />

Table 13 provides values for the estimated maximum<br />

application rates for various soil textures <strong>and</strong> ground<br />

cover under normal irrigation set times. The rates<br />

shown are assuming irrigation set times <strong>of</strong> between 4<br />

to 24 hours. Acceptable maximum application rates<br />

for irrigation set times less than this time frame may<br />

be different. The values shown below are for level<br />

ground. For sloping ground the maximum rate must<br />

be reduced. A general rule is to reduce the maximum<br />

application rate by 25% for field slopes exceeding<br />

10% <strong>and</strong> by 50% for fields with slopes exceeding<br />

20%. Field tests under actual sprinkler conditions<br />

should be conducted to determine accurate<br />

application rates. There is an increased risk <strong>of</strong> soil<br />

erosion, mass wasting or other soil water related<br />

problems when application rates exceed those given<br />

in Table 13.<br />

3.1.1.6 Maximum Irrigation Interval<br />

The maximum irrigation interval is the maximum<br />

number <strong>of</strong> days between irrigations that a crop can<br />

sustain optimum growth <strong>and</strong> production. It is<br />

calculated by dividing the maximum soil water deficit<br />

by the peak evapotranspiration rate. The actual<br />

irrigation interval may differ from the maximum<br />

irrigation interval if the irrigation system does not<br />

apply the total amount <strong>of</strong> water capable <strong>of</strong> being<br />

stored in the soil at the maximum soil water deficit.<br />

Irrigation should be applied to most crops when 50%<br />

<strong>of</strong> the water in the soil, which is available to the<br />

plants, has been depleted.<br />

3.1.1.7 Evapotranspiration Rates<br />

Evapotranspiration (ET) is a measure <strong>of</strong> the rate at<br />

which water is transpired from plants plus the water<br />

evaporated from the soil <strong>and</strong> crop surface. The peak<br />

ET averaged over the irrigation interval decreases as<br />

the interval increases. Since the irrigation interval is<br />

related to the AWSC <strong>of</strong> the soil, peak ET decreases<br />

with an increase in maximum soil water deficit.<br />

Table 14 provides the best estimates <strong>of</strong> the highest<br />

Table 13<br />

Maximum Design Application Rates (based on a soil infiltration rate when irrigation set is >4 hours)<br />

Textural Class Grass Sod (cm/hr) Cultivated (cm/hr)<br />

S<strong>and</strong> 1.9 1.0<br />

Loamy S<strong>and</strong> 1.7 0.9<br />

S<strong>and</strong>y Loam 1.2 0.6<br />

Fine S<strong>and</strong>y Loam 1.0 0.6<br />

Loam 0.9 0.5<br />

Silt Loam 0.9 0.5<br />

Clay Loam 0.8 0.4<br />

Clay 0.6 0.3<br />

Organic <strong>Soil</strong>s (Muck) 1.3 1.3<br />

50 <strong>Soil</strong> <strong>Management</strong> <strong>H<strong>and</strong>book</strong> – Okanagan-Similkameen Valleys

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