# Sustainable Management of Water Resources in Supplementary Irrigation Management
## 3 Results  
In general, the sensors' reflection of rainfall patterns over the study period was soil-depth-dependent (Figure 4). ANOVA showed a significant variation in SWC across the soil layers for all tested sensors (LSD\({}_{p<0.01}\) 34.97 for Watermark, LSD\({}_{p<0.01}\) 4.98 for tensiometer and LSD\({}_{p<0.05}\) 3.18 for TDR). Especially notable was the performance of Watermark sensor as a function of soil depth, which will be discussed later. The results of the study clearly provide insight into the interpretation of sensor readings with respect to the time of irrigation. For silty clay loamy soil at 0 to 40 cbar, Watermark and Tensiometer readings represent the FWC range, and at 40 to 70 cbar, the irrigation initiation range or management allowable depletion (MAD). This result of our study confirms that there is a need for sensor calibration for specific soil types, since according to the manufacturer's recommendation, the FWC is in the range of \(<\)20 cbar, while according to our results, it is \(<\)40 cbar, or irrigation initiation range is \(<\)80 cbar while in our study it is \(<\)60 to 70 cbar. In terms of time irrigation time and amount of irrigation water, this means that irrigation scheduling according to the manufacturer's guidelines will lead to over-irrigation. In the 8 to 12 cm soil layer, VWC ranged from 19 to 37.6% (27.9% on average). A considerable amount of rainfall occurred at the beginning of June (24.3 L, Figure 5a), which increased the SWC to almost 100% FC. In the 0 to 10 cm soil layer, the Watermark sensor and TDR captured the change in SWC, from 74 to 42 cbar and 23.3% vol. to 32.9% vol., respectively. A similar situation occurred at the end of June (16.6 L), when Watermark and TDR captured the increase in SWC from 67 to 45 cbar. For the 10 cm soil layer, there was a strong positive correlation between TDR and VWC (r = 0.93, \(p<0.05\)) and a strong negative correlation between Watermark and VWC (r = \(-\)0.83, \(p<0.05\)). Regression analysis was done to test the significance of the relationship between SWC and sensor readings. The best fit of the regression function (linear or polynomial) was chosen according to the highest R\({}^{2}\) (\(p<0.05\)). In comparison to Watermark, TDR showed better performance, which is obvious from the scatter patterns (Figure 6a,h). The scatter dots (Figure 6h) for TDR showed good distribution along the 1:1 line (R\({}^{2}=0.86\)) when SWC was below 33%. The difference in SWC between the TDR and VWC was below 2%. Watermark sensor (Figure 6a) did not show a tight distribution along 1:1 line (R\({}^{2}=0.68\)). Figure 5: Amount of rainfall (mm) and soil water content (Volumetric Water Content (VWC) (%), cbar) at 10 (a), 20 (b), 30 (c) and 45 cm (d). In the 18 to 22 cm soil layer, VWC ranged from 19.2 to 38.7% (29.2% on average). Similar to the previous soil layer, all tested sensors captured the changes in SWC (Figure 5b). In this soil layer, both Watermark and tensiometer captured SWC changes with good matching up to the point where SWC was getting lower, above 60 cbar. This indicates that both Watermark and Tensiometer measure SWC with great accuracy in the range of readily available water (0 to 80 cbar). Although Watermark had a wider measuring range (0 to 199 cbar) than tensiometer, the measurements outside 80 to 100 cbar have no meaning for irrigation scheduling. In this soil layer, the best performance had a Watermark sensor (Figure 6b) with scatter dots close to the 1:1 trendline when VWC was above 23.7%. As the SWC was decreasing, the Watermark mostly underestimated SWC. The correlation analysis showed strong negative relationship between VWC and Watermark (r = \(-\)0.96, \(p\) < 0.05), tensiometer (r = \(-\)0.83, \(p\) < 0.05) and VWC. At this soil depth, TDR generally followed the pattern well (Figure 6i), although without thigh distribution along the line. The coefficient of determination (R\({}^{2}\) = 0.86) was almost the same as in the previous soil layer, which indicates that TDR measurement is not soil-depth-dependent.