Estimating crop coefficients from canopy cover and height for a drip-irrigated young almond orchard: assessment using a two-source energy balance model

The aim of this study was to estimate standard crop coefficients of surface and sub-surface drip-irrigated young almond trees under non-limiting soil water content conditions, based on measurements of the fraction of ground covered by the canopy (fc) and tree height (h) (A&P approach proposed by Allen and Pereira (2009)) and to improve the transferability of them to the productive sector once weather data (i.e. maximum and minimum air temperature (Tmax and Tmin, respectively), as well as dew point temperature (Tdew)) were adjusted to the reference conditions. A 4 year field experiment was carried out in a ~ 12.5 ha commercial young almond (Prunus dulcis (Mill.) D.A. Webb) orchard located in Hellín, (SE Spain). ‘Penta’ almond trees, grafted onto the GF-677 rootstock, were planted in 2018. Field measurements of fc and h were performed over four consecutive growing seasons from 2019 to 2022. In parallel, ETo computed by the nearest meteorological station, located at a non-reference weather site, was reduced around 6% after bringing weather data closer to the reference conditions, while actual crop evapotranspiration and its components (actual tree transpiration and soil evaporation) were estimated in each irrigation system through the so-called simplified two-source energy balance (STSEB) model in order to be used as a quality assessment of the A&P approach. The ratio between the former estimations and the ETo allowed to compute STSEB-based crop coefficients. No significant differences in effective canopy cover (fc eff) nor h were observed between the two irrigation systems, and thus the estimated Kcb values were the same for both drip-irrigation systems. fc eff values during mid-season stage ranged between 0.15 in 2019 and 0.62 in 2022, whereas average h values for this stage ranged between 2.36 and 3.80 m in 2019 and 2022, respectively. These values of fc eff and h resulted in average mid-season basal crop coefficients (Kcb-mid) of 0.28 in 2019, 0.39 in 2020, 0.61 in 2021 and 1.02 in 2022. Soil evaporation estimates through STSEB model were significantly different between the two irrigation systems, leading to differences in Ke being around 16% higher for DI than SDI. Moreover, the intra-annual Kc values moved in the same range for the initial, mid- and end-season crop growth stages, varying from 0.30 in 2019 to 1.01 in 2022, computed under standard conditions. Finally, the A&P approach was shown to be an especially interesting method for estimating Kcb values in almond fruit trees, being useful for refining Kcb and/or Kc for conditions of plant spacing, size and density that may differ from standard values. In this way, irrigation scheduling can be optimized regarding the almond tree architecture (i.e. fc eff and h), allowing to manage properly irrigation water to meet the tree water demands.

This work was funded by the Education, Culture and Sports Council (JCCM, Spain) (project SBPLY/21/180501/000070) and the Agencia Estatal de Investigación (project TED2021–130405B-I00), together with FEDER and Next Generation EU/PRTR co-financing. F. Montoya thanks the JCCM (Spain) for the support to develop the SIMA project (SBPLY/21/180501/000152).

Peer reviewed