Sprinkler Distribution Uniformity for Strawberry Cold Protection

Effective sprinkler irrigation for cold protection requires an adequate and uniform water application rate (AR) in order to provide enough heat by freezing to compensate for heat loss by other means (i.e., evaporation). Several models have estimated the sufficient AR for cold protection as $6.35\mathrm{mm}/\mathrm{h}$; however, this value has little field verification. The objective of this study was to evaluate the effect of sprinkler type, sprinkler spacing, and water supply pressure under varied wind conditions on low quarter irrigation distribution uniformity ($D{U}_{lq}$) and AR. Distribution uniformity was tested using four sprinkler types (Wade Rain WR-32 and Nelson: R33, R33LP, and R2000WF) assessed at three pressure levels (345, 276, and 207 kPa), two sprinkler spacings (14.6 and 12.2 m), and varied wind conditions ranging from low ($0.2\mathrm{m}/\mathrm{s}$) to high ($7.2\mathrm{m}/\mathrm{s}$). The interactions (1) sprinkler type–pressure, (2) sprinkler type–spacing, and (3) pressure-spacing resulted in a significant effect on $D{U}_{lq}$ and AR, as well as, the effect of wind speed, which was included as a covariate in the analysis. The R2000WF and WR-32 sprinklers at 345 kPa and 12.2 m spacing resulted in the highest uniformity, and by contrast, obtained the lowest AR when evaluated at 207 kPa and 14.6 m spacing. Overall $D{U}_{lq}$ did not differ drastically across the factors tested, whereas AR was substantially impacted by these factors. Current strawberry growers systems are set up at 14.6 m sprinkler spacing with $2–3\mathrm{mm}/\mathrm{h}$ AR, which is substantially lower than the recommended AR of $6\mathrm{mm}/\mathrm{h}$ and may result in inadequate cold protection. Using manufacturer recommended spacing with commonly used sprinklers (WR-32) would result in $4–5\mathrm{mm}/\mathrm{h}$ AR depending on supply pressure available; hence, a greater level of cold protection.