# Sustainable Management of Water Resources in Supplementary Irrigation Management
###### Abstract  
Tensiometer and Time Domain Reflectometry (TDR) are commonly used soil water sensors in irrigation practice whose performance depends on soil type, depth and growing conditions. Here, the results of sensor performance evaluation in different soil depths as well as the field and laboratory testing in silty clay loamy soil are presented. Gravimetric soil moisture samples were taken from sensor installation depths (10, 20, 30 and 45 cm) and used as reference Soil Water Content (SWC). The measurements varied significantly (\(\text{r}<0.05\)) across the monitoring depths. On average across the soil depths, there was a strong negative linear relationship between Watermark (\(\text{r}=-0.91\)) and TDR (\(\text{r}=0.94\)), and a moderate negative (\(\text{r}=-0.75\)) linear relationship between SWC and Tensiometer. In general, Watermark and Tensiometer measured SWC with great accuracy in the range of readily available water, generated larger Mean Difference (MD) than TDR and overestimated SWC, while TDR underestimated SWC. Overall, laboratory testing reduced the root mean square error (RMSE, Watermark = 1.2, Tensiometer = 2.6, TDR = 1.9) and Mean Average Error (MAE, Watermark = 0.9, Tensiometer = 2.04. TDR = 1.04) for all tested sensors. sensor performance; field testing; laboratory testing; statistical analysis +
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water content of PWP (\(-1.5\) MPa). The PWP is the lower soil water limit below which plants cannot effectively extract water and will eventually wilt. The key factor in successful irrigation scheduling is to know the RAW that represents the Maximum Allowed Water Depletion (MAD) at the time of irrigation. There are different techniques to determine the time of irrigation and how much water to apply in one irrigation event. In practice, this decision is mostly based on personal experience of the irrigation engineer or meteorological data or by counting the calendar days since the last irrigation, while the most reliable and precise technique for irrigation scheduling is measuring and monitoring of Soil Water Content (SWC) during the growing season. In practice, some of the sensors for measuring SWC are more reliable or have higher maintenance requirements than others. Challenges in the measurement of SWC include labor-intensive activities such as the permanent installation of soil moisture devices, repeated monitoring of water content, destructive sampling, and water extraction from soil samples [4]. SWC can be measured directly (gravimetric, destructive sampling) or indirectly with the use of sophisticated devices (in situ sampling). [PERSON] [5] has stated that destructive sampling, which involves gathering soil samples in the field and taking them to the laboratory for analysis, can provide good measurements of SWC but can be time-consuming and does not allow for retesting at the same site due to removal of the soil material. Furthermore, the author claims that in situ sampling gives results much more rapidly than destructive sampling and can provide repeated measurements from the same site at virtually any time interval desired, given the correct data logging equipment, but in situ instrumentation is typically quite expensive in comparison to destructive sampling. The in situ use of sophisticated expensive sensors for measuring SWC is not often used in practice, mostly due to a lack of previous knowledge about SWC and of course due to its high price, even though the measuring of SWC is essential for irrigation scheduling by keeping it within a target range. Regarding previous knowledge of SWC, [PERSON] et al. [6] have stated that prior knowledge of the FWC and the soil-water retention curve (also named as the soil moisture characteristics curve) is important for the effective irrigation management and scheduling of many crops. [PERSON] [7] claims that the soil-water retention curve is affected by soil physical and chemical characteristics, while [PERSON] et al.