Laserfiche WebLink
this relationship: <br />qET MET - {iET/(tET'(0 -6))} <br />c7 _ � 1 t t (6) <br />qGw lttitt —KC — {iRC /('t RC' t <br />J ET✓(tRC'(� —e)) <br />where <br />mET = rate of groundwater -level recession from ET <br />mRC = rate of groundwater -level recovery from ET <br />iET = total infiltration that occurs during the ET portion of the daily cycle <br />iRC = total infiltration that occurs during the recovery portion of the daily cycle <br />tET = length of time during which ET occurs <br />tRC = length of time during which recovery occurs <br />(0 -6) = air - filled porosity of the soil immediately above the water table <br />The total ETwT in inches per day can be calculated from the qET rate by multiplying qET times <br />the duration of ET for the day (tET). Most of these terms (mET, mRC, tET and tRC) can be <br />measured directly from the groundwater -level recording chart. The infiltration terms (iET and <br />iRC) and the air - filled porosity must be estimated, however. Infiltration is usually <br />negligible, except for periods of precipitation. Without a more precise measurement, Gerla <br />(1992) suggests using the total precipitation for the period as an estimate for infiltration. This <br />assumes little or no runoff and that interception by plant foliage is negligible. Both <br />assumptions are probably valid for the sandy Platte River soils and for the larger precipitation <br />events, which tend to make runoff and interception insignificant compared to infiltration. <br />Gerla (1992) also suggests dividing the infiltration (i.e.. precipitation) by the change in <br />groundwater level to obtain a rough estimate for (0 -8). This technique seemed to work well <br />most of the time for our study, but usually produced widely different values from one event to <br />another. Most of these differences can probably be attributed to runoff from saturated or <br />frozen soil, 'excessive losses' to the dewatered soil above the capillary fringe, or the spatial <br />distribution of precipitation between the gage and the well. To reduce this variability between <br />23 <br />