Update 2012: Soil Water Content Monitoring
The spatial and temporal variation of soil water content in the root zone for the drip site is shown in Figure 2. The black and red arrows indicate the irrigation and rain events during the presented time period, respectively. The (X, Y) notation represents the Cartesian coordinate system, with both X and Y, representing distances (cm) from the tree trunk. For example, the panel with the (0,150) notation presents soil water content data that is exactly along the tree row (X = 0 cm) and midway between the trees (Y = 150).
We notice differences in soil water content variations with time between the wet (winter and early spring), and dry (summer and fall) seasons. After each infiltration event in winter, the infiltrated water moves through the sandy loam layer until reaching the clayey loam layer, after which deeper wetting is stagnated and delayed by the finer-textured soil layers by a few weeks. During this time of redistribution, soil water content above the restricted soil layer decreases to field capacity, whereas the deeper soil layers increase their water content gradually. For the summer period, evapotranspiration demand is high and the trees require frequent irrigation.
These irrigations result in much larger variation of soil water content at the shallower soil depths where most of the tree roots are located. The near constant water content values at the larger depths below the finer-textured layers are an indication of little or no leaching. Figure 2 shows that the soil water content at the shallow depths below the berm at panels (0,150) and (0,300) did increase after rainfall, but not during irrigation events. Instead of infiltrating along the drip line, much of the applied irrigation water would flow downhill over the berm surface towards the berm edge and infiltrate there instead.
The spatial and temporal variation of soil water content in the root zone for the fanjet irrigation site is shown in Figure 3. The same pattern of deep percolation during the wet period was observed for the fanjet site. Although patterns are similar, the changes in water content at deeper depths are slightly different. Specifically, we note that the 180-cm deep water content readings are much lower for the fanjet than for the drip site (Fig. 2), likely because of the coarser soil texture of the fanjet site at that depth (compare soil layering between sites in Figure 1). Also, because of the larger wetted area of the fanjet system, wetting patterns were different between sites, and shallow sensors responding to infiltrating applied irrigation water in the berm for panels (0,150) and (0,300).
Fig. 2. Spatial and temporal variations of soil water content in the root zone under the drip irrigation system. The black arrows indicate the irrigation events and the red arrows denote the precipitation events.
Fig. 3. Spatial and temporal variations of soil water content in the root zone under the fanjet irrigation system. The black arrows indicate the irrigation events and the red arrows denote the precipitation events.
For more information see the presentations and publications under Outreach