Weighing lysimeters for measuring evapotranspiration and developing crop coefficients for trailing blackberry

Precision weighing lysimeters are expensive but invaluable tools for measuring crop evapotranspiration (ET_c) and developing crop coefficients (K_c). We installed two of this type in Oregon, USA and planted them and the surrounding field with ‘Columbia Star’ trailing blackberry (Rubus L. subgenus Rubus Watson). The lysimeters were constructed from steel and included an inner tank (1.0 m wide × 1.5 m long × 1.7 m deep), resting on four shear-beam load cells, inside an outer tank. During the first year after planting (primocanes only), ET_c totaled 120 mm, or 18% of total potential evapotranspiration (ET_o). That year, K_c values, calculated by dividing ET_c by ET_o, was 0.24 when the measurements were initiated in early July and increased to 0.81 before leaf fall in early October. The following year, ET_c doubled due to floricane and primocane growth, reaching a total of 275 mm, or 34% of total ET_o. At that point, K_c increased from 0.30 at budbreak to 0.48 before it dropped to 0.27 due to leaf damage that occurred due to record-breaking heat in late June. Afterwards, K_c slowly increased with primocane development until it reached 0.85. By the third year, the plants reached full production, and ET_c doubled again, reaching a total of 511 mm, or 71% of total ET_o. Weekly K_c in year 3 increased from 0.55 in late June to 0.88 in mid-August, where it remained until the plants were pruned and trained in late September. At full production, the plants required an average of 2 L of water (rain or irrigation) to produce each individual fruit and over 48 L of water to produce enough berries to fill a 170-g clamshell. The plants extracted water as deep as 1.2 m, indicating that it is important to maintain adequate soil moisture to at least this depth in trailing blackberry.