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Fluid Journal : Summer 2014
16 The Fluid Journal Summer 2014 (what is now known as an “allowable deficit”). And lastly, there are those who think the lower limit should be set at the point where the first slowdown in water use by the crop occurs. The truth is, no matter which of the three was used, it always improved water use efficiency. All reduced applied water volumes and prevented waiting too long to irrigate. Water efficiency To debate which method is best maybe misses the real issue of water efficiency. Let’s first define water efficiency. The simplest method is to add all rainfall plus the amount of irrigation the crop receives to get total applied water, then divide the total yield by that number (yield/ total water applied). We would argue this is not a good method for the calculation, for reasons we will explain below. The next interaction would be to add the net change of the Starting Volumetric % Water Content and Ending Volumetric % Water Content of soil water content of the soil water [(Y/TWA) + (SWC - EWC)]. This takes into account how the water in the soil changed over the season. If EB is higher at the end, then the plant did not use all the applied water and the water efficiency of the plant should get that credit. This all makes sense but what about large rainfalls? Did the plant use all that water? When a good manager gets the large rainfall, did he really mismanage the water and cause the water efficiency to go down? Most likely not, especially if there was runoff and the soil doesn’t have a high water holding capacity. So how can we account for this in our measurement of water efficiency? Acknowledge that big rains serve more purposes than just for the growing of crops, i.e . recharging groundwater and surface flows. And how should we change the efficiency formula? How about by accounting the amount of rainfall to only the volume used by the crop?! In the past this could be at best a guess, but now, with technology, it possible to measure that consumption. Main forces There are four main forces moving water in the soil. Drainage (or percolation) is the water loss from the soil macropores and as gravity pulls that water deeper into the soil profile, it is replaced by air. Drainage is a downward movement. Evaporation is the soil water loss due to sunlight and wind, and the water is lost off the surface of the soil into the air above the soil surface. This is upward movement of water. Transpiration is the water that enters the roots, moved upward through the plant, and evaporates from the leaves through the stomata (leaf pores). This is water leaving the soil (generally, below the soil surface) and going out to the atmosphere. This is the water that is used by the crop. Normalization is the movement of water from areas of high concentration (wet areas) to areas of low concentration (dry areas) by diffusion. This can be in any direction: up, down, sideways, or any combination. Depending on the soil texture and the current water balance, each of these forces moves water at different velocities, with drainage (powered by gravity) as the most rapid, and transpiration as slower than gravity, but faster than evaporation and normalization, which are the laggards in the pack. Due to the difficulty to distinguish between evaporation and transpiration, we mostly talk about ETo (the combined effect of the two forces of evaporation and transpiration). But Etc (plant coefficient of water uptake by the crop) is what we really need to know in order to talk about plant water efficiencies. There have been several ways the crop consumptive use has been measured (or calculated). The most straightforward method is to grow plants in a container (that can be weighed periodically or with lysimetry, which involves large soil cores and sophisticated weighting methods). The basic premise is that the weight of water added can be measured, and then, as the water leaves the plant, the total weight of the container or soil core will go down; the difference is the water transpired (used by the Figure 1. The green area is the desired water level. Figure 2. The decrease in lines shows water leaving the soil quickly until field capacity is achieved.