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Sometimes `check flood' systems are combined with `corrugated' or `bedded' systems which <br />facilitate water movement and drainage on heavy soil. <br />A description of the behavior, of flood irrigation is as follows:. <br />• At the start of the irrigation, the water starts flowing or advancing down the check. <br />• At the same time, water ponds on the soil surface. During the irrigation, the amount of <br />ponded or stored water is substantial. The ponded water can be at least 3 to 4 inches deep. (Note: <br />lit contrast, stored water during furrow irrigation is insignificant relative to the amount applied.) <br />• The ponded water infiltrates the soil as water flows across the field. <br />• At cutoff, the irrigation water is stopped. The ponded water, however, continues to flow down <br />the field. The ponded water continues to infiltrate the soil after cutoff and may supply all of the <br />soil moisture replenishment along the lower part of the field. <br />* The stored water also causes surface runoff at the end of the field. The longer the cutoff time, <br />the more the runoff from the stored water. <br />The rate at which the water flows down the field depends on the inflow rate of water into the <br />check, slope, length of the border <br />check, soil infiltration rate, and <br />surface roughness. The flow of 26 <br />water across the field is characterized by the advance 20 Recession Curve <br />curve, which shows the time at s <br />which water arrives at any given 1s <br />distance along the field length (Fig. inriiratton rime <br />Another characteristic curve is the <br />recession curve, which shows the <br />time at which water no longer <br />ponds on the soil surface at any <br />given distance along the field <br />length (Fig. 1). Water recession <br />after cutoff' depends on the length <br />of the border, slope, surface <br />roughness, and infiltration rate. <br />io <br />s <br />0 200 4oo wo 809 1000 1200 1400 1500 <br />Distance Along the Field (feet] <br />Figure 1. Advance and recession curves for a flood-irrigated <br />alfalfa field. <br />The difference between advance times and recession times is the time during which water <br />infiltrates the soil or the infiltration time. These infiltration times vary along the field length, <br />resulting in more water infiltrating in some parts of the field compared to other areas. <br />Advance curve <br />89