236 Nielsen et al.: Soil Water Dynamics and Growth of Trees 208 and day 235). In the second dry period, the differences between soil horizons in the park soil were not significant. The seasonal fluctuations in soil water are smaller in the 25 to 50 cm (10 to 20 in) soil horizon than in the surface horizon (Figure 4). The standard deviations from the four samples, which are plotted in Figure 4, were 10% and 12% for the 25 to 50 cm (10 to 20 in) horizon and 19% and 23% for the surface horizon for street planting pits and park soil, respec- tively. Effect of Irrigation Treatments and Site on Soil Water Irrigation effects were analyzed both in terms of the volu- metric water content and in terms of the soil water saturation index. Significant responses to the irrigation treatments were found after all treatments except for the first irrigation on day 147 (26 May) (Table 3). Figure 5 shows the treatment effects related to the control plot. For simplicity, the figure is based on water assessments from seven of 18 campaigns. Four campaigns within the treatment period with the highest (days 208 and 241) and the lowest (days 166 and 223) soil water saturation levels were selected for illustration of the treatment effects. All treat- ments in Figure 5 resulted in improvement of the soil water status. The irrigation treatments of 640, 320, 280, and 140 L water (166.4, 83.2, 72.8, and 36.4 gal) caused the soil water level to be on average 26%, 18%, 7%, and 4%, respectively, above the control during the treatment period. The park soil—after a very dry period during May/June—continuously and significantly improved its water status in comparison to the street tree pits (Table 3). This is confirmed in Figure 4, showing that the soil water saturation index at a depth of 25 to 50 cm (10 to 20 in) has a significantly increasing trend line in the park soil and a significantly decreasing trend line in the street tree planting pits. Time Domain Reflectometry Responses Immediately After Irrigation At five different times during the project, the soil water con- tent was measured 1 day before and 1 day after the irrigation (on irrigation day 239, the soil water assessment was carried out 2 days after the irrigation). The absolute water content (in liters) was calculated from TDR data in the upper 25 cm (10 in) soil layer before and after the irrigation. Consequently, an increase in soil water was estimated from the TDR assess- ments. In theory, this increase should reflect the amount of irrigated water. Figure 6 shows the TDR-estimated increase in soil water in a planting pit after irrigation as a function of the added amount of water. If the change in estimated soil water pre- cisely reflected the irrigated amount of water, all observations should be on the solid reference line in Figure 6 with a slope of 1.0 (i.e., estimated soil water change is identical to the amount of water that was added). However, the TDR- estimated soil water change after irrigation treatments tends Figure 5. The deviation of the treatments from the street tree control is calculated on the basis of the saturation index. The park soil assessments are also shown as a fur- ther reference. The vertical dashed lines show the begin- ning and end of the irrigation treatment. ©2007 International Society of Arboriculture Figure 6. This analysis questions whether the time domain reflectometry (TDR) assessments on the day after irriga- tion reflect the amount of added water to the street tree planting pit. Five irrigation events are analyzed. The thick, straight reference line is where Y = X. On days 162, 197, 215, and 224, the irrigation response was measured on the next day. On day 239, the response was measured 2 days after irrigation. The figure shows that overestimation happens when TDR measurements are carried out on the first day after heavy water input.
July 2007
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