Modeling stand water use response to soil water availability and groundwater level for a mature Populus tomentosa plantation located on the North China Plain

- Crop coefficient models for predicting water use in Populus tomentosa plantations were constructed and tested.
- Crop coefficients reached a maximum at relatively low LAI, compared with other reported values.
- Soil water in various layers from 0 to 70 cm depth contributed differently to variation in transpiration.
- Reduction in transpiration occurred when rθ in 0-30 cm soil reached a threshold of 0.6.
- The critical threshold of groundwater level for full transpiration was 300 cm depth.

To help achieve precise irrigation in Populus tomentosa plantations, the stand water use characteristics of a mature P. tomentosa plantation under well-watered drip irrigated conditions were investigated over two growing seasons (April-October) in 2010 and 2011. Crop coefficient models for predicting stand water use were constructed and tested. The quantitative responses of stand water use to groundwater level (GWL) under different irrigation conditions and soil water availability (rθ) in different soil layers, which have not been thoroughly examined in poplar plantations, were also investigated. The stand evapotranspiration (ETa) was dominated by soil evaporation (Es) before late April and after the middle of September, but transpiration (Tr) became the dominating component of ETa between late April and mid-September accounting for 77%. The mean daily Tr, Es and ETa for non-rainfall periods were 2.67, 1.04 and 3.71 mm d−1, respectively. The relative mean absolute error of the crop coefficient models used to predict ETa (13%) and Tr (16%) in the non-irrigated treatment (CK) during periods with no water stress were small, suggesting these models can be used to accurately predict stand water use of P. tomentosa under well-watered conditions. Fractional transpiration rate of P. tomentosa was significantly (p < 0.0001) correlated to rθ of different soil layers within 0-70 cm depth, but the same relationship was not detected for rθ below 70 cm depth. The proportion of variation in Tr explained by rθ was highest (R2 = 0.630) in the 0-30 cm layer. Tree transpiration was unconstrained when the rθ of the 0-30 cm layer was above 0.6, but if rθ in the surface 30 cm soil was not maintained above 0.6, there was a reduction of water uptake and transpiration in P. tomentosa. Significant (p < 0.0001) correlation was found between fractional transpiration rate and GWL in the CK treatment, and as the GWL decreased below 300 cm depth, Tr of P. tomentosa declined gradually. Whereas, similar phenomenon was not observed in the irrigation treatment. This study therefore indicates (1) that the shallow soil layers should be the key soil zone for irrigation water management in plantations of P. tomentosa and similar tree species located on sites similar to those in our study, and (2) as the GWL declines below 300 cm depth, irrigation should be applied in P. tomentosa plantations to maintain water uptake and tree growth.