Irrigation in the Pacific Northwest

Woodruff Charts. Graphical paper & pencil irrig. sched.

The Woodruff Chart uses historic average weather data and the 5 point crop coefficients (as in FAO 56) to create a chart that can be used for graphically doing irrigation scheduling. The top line is the cumulative average crop water use at the selected station. The bottom line shows when water stress would begin to occur and is offset a vertical distance equal to the readily available water holding capacity of the soil. The X- axis (horizontal axis) is time (days). To schedule irrigations, the user draws a vertical line as long as the rainfall and/or irrigation amounts on the day they occur. The following rainfall or irrigation events are also drawn on the graph on their corresponding date, but start at a level equal in elevation to the top of the last drawn-in rainfall or irrigation event. This will create a stair-step pattern throughout the season. Irrigation should be scheduled before the horizontal stair-step crosses the lower water stress line. If a rainfall or irrigation amount would take the stair-step across the top line, any portion above the top accumulative water use curve is considered deep-percolation, and is discounted. In this case, the irrigator begins the horizontal stair-step, not at the top of the last irrigation line, but at the location where the irrigation or rainfall line crosses the top cumulative water use line. (-Adapted from J.C. Henggeler, Univ. of Miss.)
Select Crop:Irrigation Efficiency
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