ET-IDWR

Statistics on six types of of water consumption parameters were produced for this station and land cover (crop type):

a) actual evapotranspiration;
b) potential evapotranspiration;
c) basal evapotranspiration;
d) precipitation deficit (i.e., net irrigation water requirement);
e) effective precipitation (within the root zone) used for supporting transpiration and evaporation

The actual daily ET_c (ET_act) represents the total estimated flux of ET given any reduction in potential ET caused by soil water shortage or soil surface dryness. ET_act was computed as ET_act = K_s * ET_bas + K_e ET_r where K_s is a stress factor (0 – 1 where 1 means no stress), K_e is the evaporation coefficient and ET_r is the alfalfa reference ET. ET_bas is defined below. ET_act was often less than potential ET (ET_pot) for rainfed crops and occasionally for irrigated crops prior to the growing season when a low-level, basal crop coefficient assigned to the nongrowing season cover could not be sustained by precipitation, and, therefore, the actual ET, in the form of mostly evaporation, could not be sustained at the ET_pot level. On occasion, ET_act fell below ET_pot early in the growing season prior to initiation of irrigation for the same reasons. ET_act includes evaporation from the soil surface from both precipitation and any simulated irrigation. ET_act is generally the most appropriate ET parameter to use for formulating estimates of consumptive use of irrigation water for hydrologic studies.

The potential daily ET_c (ET_pot) represents the total estimated flux of ET that would occur if there were no moisture stress imposed by soil water shortage in the ‘root zone.’ ET_pot includes evaporation from the soil surface from both precipitation and any simulated irrigation. ET_pot was computed as ET_pot = ET_bas + K_e * ET_r.

The basal ET (ET_bas) represents the ET that would occur under no water stress and with no surface wetting by precipitation or irrigation. In other words, ET_bas represents potential ET for a dry soil surface. ET_bas should not be used to estimate irrigation water requirements, and is included to provide an indication of the partitioning of ET_pot between ‘transpiration,’ as represented by ET_bas and evaporation of water from the soil surface layer. (K_cb)

The precipitation deficit (P_def) is the difference between the potential ET (ET_pot) and the amount of precipitation infiltration and residing in the root zone. In essence, P_def, is the amount of additional precipitation needed to fully support ET_pot. P_def was calculated as ET_pot - P_rz where P_rz is the precipitation infiltrating into and residing in the root zone. P_def is synonymous with the net irrigation water requirement when occurring during the growing season for an irrigated crop. P_def represents the amount of additional water that the crop would evaporate or transpire beyond P_rz if that water were made available at the right time during the growing season. Because the ET_pot estimate only includes soil evaporation for known precipitation and irrigation events, any additions of P_def outside of the simulated growing season would tend to increase ET_pot and thus P_def to some extent, causing a type of positive feedback process. The precipitation deficit is generally the most appropriate "ET" parameter to use for irrigation system design.

The effective precipitation residing with in the root zone used for transpiration and evaporation represents the precipitation that enters the root zone and is used in supplying transpiration from the vegetation and evaporation from the soil at some point in time after the precipitation event.