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294 D.C. Goodrich et al. /Agricultural and Forest Meteorology 105 (2000) 281 -309 (PptH,$) was also derived by scaling the rainfall mea- sured at the mesquite tower by the remotely sensed es- timate of stream surface area. As in the case of EWs, a large uncertainty of 40% was assigned to the remotely estimated water area. Because rainfall typically has a high degree of spatial variability, a 40% error was assigned to the measured rainfall depth. Precipitation falling on vegetation in the riparian corridor was not considered in the water balance. None of the rain- fall events recorded during DOY 101 -191 exceeded 5 mm with largest being 4.3 mm over a 4.34 h period. It was assumed that this light rainfall was intercepted by vegetation or fell on the soil surface such that it quickly evaporated and did not substantially increase soil moisture. However, the observed rainfall amounts were assumed to satisfy ET demands of the vegeta- tion. The rainfall totals were subtracted from the to- tal estimates of transpiration from the C/W and the mesquite. The net volume of groundwater inflow (GWnet) from the regional aquifer (deep basin sediments) was determined by computing the gains during a period of no transpiration and then indexing that gain by the water surface potential (head) gradient between a deep well in the regional aquifer and a shallow well at the top of the regional aquifer. Indexing to the head gradient was assumed to account for changes of inflow over time into the control volume from the regional aquifer (see middle portion of Fig. 4). By selecting a dry pre - green -up period, the riparian ET terms are assumed to be zero. If the selected period of time has no rainfall and assuming the residual term is zero, Eq. (6) can be solved for GWne1 as GWnet = Qout — Qin + EWs + OStorage (8) By utilizing field observations and examination of the stream discharge and groundwater level data avail- able, the period DOY 80-90 was selected to compute GWnet, Qin, and Qout• Evaporation from the stream water surface (EWs) was computed as discussed above. The change in storage term results from a draining of the alluvial aquifer, a decrease in stream stage, and gains or losses of unsaturated soil moisture. The change in storage was computed for three portions of the control volume as OStorage = OSpost + OSpre + OSst + OSM (9) In this equation, OSpost is the change in storage of the post- entrenchment alluvial aquifer which lies in a nar- row portion of the riparian floodplain adjacent to the currently active channel (see middle portion of Fig. 3). The extent of this aquifer was obtained from Demsey and Pearthree (1994). OSpCe is the change in storage within the pre- entrenchment alluvial aquifer. The area of this aquifer was determined by subtracting the area of the post- entrenchment aquifer from the overall ri- parian corridor area described in Appendix A (also see middle portion of Fig. 3). OSst is the change in storage within the stream. This was computed by multiplying the remotely sensed stream area by the average change in stage at Lewis Springs and Charleston at the begin- ning and end of the DOY 80-90 time period. ASM is the change in unsaturated soil moisture. For the DOY 80-90 time period, this term was assumed to be zero as groundwater flow was upward into the alluvial aquifer and largely disconnected from the unsaturated zone that was monitored for soil moisture changes. OSM was considered in the DOY 101 -191 water balance as it could readily contribute to transpiration and evapo- ration losses out of the control volume. Readings from water content reflectometry probes at depths of 10, 25, 50 and 100 cm in soils near the mesquite and sacaton Bowen ratio towers were used to estimate changes in soil moisture (Scott et al., 2000). The change in storage of the post- and pre- entren- chment aquifers was computed by multiplying the area of these aquifers times the drop in water table within each aquifer times the specific yield of the aquifer. The specific yield is the volume of water re- leased from a porous media for a unit drop of the water table per unit area. The specific yield for the pre - entrenchment alluvium was set to 0.15 following Corell et al. (1996). The coarser material making up the post - entrenchment alluvium justified a higher spe- cific yield. A value of 0.25 was selected based on the optimized value of 0.2 obtained by MacNish et al. (2000). The 0.2 value represented an average spe- cific yield for the Lewis Springs area. The intensive measurement area at Lewis Springs contained roughly equal areas of post- and pre- entrenchment alluvium. The specific yields could be obtained with more ac- curacy with a pump test, but this was not feasible at Lewis Springs due to limited depth of saturated thick- ness within the alluvial aquifer. This was not consid- ered a critical shortcoming as MacNish et al. (2000)