A modified STIC model for estimating crop evapotranspiration

A modified STIC model for estimating crop evapotranspiration

A novel application of the STIC model (Mallick et al. 2014, 2015a) was developed by the author to estimate crop evapotranspiration, E, at a field or sub-field scale without recourse to any remote sensing (RS), i.e. all sensors were mounted near to the ground. This new ‘Ground-Proximal STIC’ (GPSTIC) system was evaluated against a Bowen Ratio Energy Balance (BREB) micro-meteorological system.

GPSTIC could make continuous estimates of E, day and night, and avoided problems associated with RS, such as limited spatiotemporal resolution, impacts of cloud cover, and intermittency of satellite overpasses.

The GPSTIC system was deployed into Australian irrigated cotton fields (118 - 185 ha) on three occasions (featuring partial canopy, bare soil, and full canopy conditions) for a total of 592 hours over the 2018/19 and 2019/20 summer seasons.

A five-height Profile BREB system, also developed by the author specifically for this research, was co-located with GPSTIC in the field. The Bowen ratio was determined from the slope of the linear regression of the T vs. e plot. The Profile BREB system included a novel algorithm that accounted for the measurement uncertainties of T and e when assessing whether to include each (e, T) point in the linear regression.

Simultaneous 60 s measurements of environmental variables were made by the independent GPSTIC and Profile BREB systems, and 4 min averages were recorded. Thus 8880 modelled values of E were made by each of the GPSTIC and Profile BREB systems.

The results showed very good alignment between GPSTIC and Profile BREB. For the three field deployments the total accumulated values (daytime and nightime data) of EGP ST IC and EBREB were, respectively, 31.2 mm and 31.1 mm, 37.6 mm and 37.6 mm, and 51.2 mm and 50.6 mm. The accumulated discrepancy between GPSTIC and Profile BREB was never larger than 2 mm.

Advantages of this new GPSTIC system over existing technologies include its ease of use and deployment; a small number of simple and inexpensive sensors (relative to other systems such as eddy covariance); low power requirements; no need for a reference crop; and no need for complex post-processing of data. GPSTIC has potential to provide good quality, continuous, real-time, low-cost E data to irrigators and researchers.