Evaluation of MOD16 Algorithm over Irrigated Rice Paddy Using Flux Tower Measurements in Southern Brazil

Abstract

Evapotranspiration (ET) is an important component of the hydrological cycle. Understanding the ET process has become of fundamental importance given the scenario of global change and increasing water use, especially in the agricultural sector. Determining ET over large agricultural areas is a limiting factor due to observational data availability. In this regard, remote sensing data has been used to estimate ET. In this study, we evaluated the Moderate-Resolution Imaging Spectroradiometer (MODIS) land surface ET product estimates (hereafter MOD16 ET -MODIS Global Terrestrial Evapotranspiration Product) over two rice paddy areas in Southern Brazil, through the ET measured using the eddy covariance technique (hereafter EC). The energy balance components were evaluated during fallow and flooded seasons showing latent heat flux dominates in both seasons. The results showed that MOD16 ET underestimated EC measurements. Overall, the RMSE (root mean square error) ranged between 13.40 and 16.35 mm 8-day −1 and percent bias (PBIAS) ranged between −33.7% and −38.7%. We also assessed the ET (measured and estimated) main drivers, with EC yielding higher correlation against observed net radiation (R n ) and global radiation (R g ), followed by air temperature (Temp) and vapor pressure deficit (VPD), whilst MOD16 ET estimates yielded higher correlation against leaf area index (LAI) and fraction of photosynthetically active radiation (f PAR). The MOD16 algorithm was forced with meteorological measurements but the results did not improve as expected, suggesting a low sensitivity to meteorological inputs. Our results indicated when a water layer was present over the soil surface without vegetation (LAI around zero), the largest differences between EC measurements and MOD16 ET were found. In this period, the expected domain of soil evaporation was not observed in MOD16 ET physical processes partition, indicating the algorithm was not able to detect areas with high soil moisture. In general, the MOD16 ET product presented low accuracy when compared against experimental measurements over flooded rice paddy, suggesting more studies are necessary, in order to reduce uncertainties associated to the land cover conditions.

Evapotranspiração De Referência Pelo Método De Penman-Monteith Em Anos De Enos Para Microrregião De Pelotas-Rs

“…Tais resultados podem ocorrer devido às características do fenômeno na região, onde o El Niño é predominantemente mais chuvoso (BOEIRA et al, 2017) e com isso, podendo resultar numa menor evapotranspiração. Como uma das principais atividades econômica da região trata-se do cultivo de arroz pelo sistema de irrigação por inundação, uma ferramenta chave para o manejo eficiente do cultivo é a estimativa da evapotranspiração, sendo fundamental para a gestão de recursos hídricos (SOUZA et al, 2019). Com isso, o presente estudo apresentou para os 480 meses de evapotranspiração estimados, a média diária de evapotranspiração de referência para Microrregião de Pelotas foi de 4,82 mm dia -1 .…”

The Fallow Period Plays an Important Role in Annual CH4 Emission in a Rice Paddy in Southern Brazil

“…Leaf area index (LAI) data were obtained from the MOD15A2 product of the MODIS (Moderate Resolution Imaging Spectroradiometer) with an 8-day temporal resolution and 1 km spatial resolution. Souza et al [38] compared LAI (MOD15A2) data with experimental measurements at the same Cachoeira do Sul site and reported similar seasonal behavior in most of the rice-growing season.…”

“…Soil water content (SWC) was measured using a reflectometer (CS616 water content reflectometer, Campbell Scientific Inc., Logan, UT, USA) at 30 cm depth. More details on the experimental site, data acquisition, and data processing are described in Diaz et al [35] and Souza et al [38].…”

“…The soil heat flux (G) was positive in almost the entire growing season, which suggests that the surface layer warms the subsoil, while the subsoil warms the surface layer in the fallow period (G < 0). Climatically, precipitation is well distributed throughout the year in the study region [35,38], varying from 83.9 mm (April) to 157.4 mm (August). On 22-23 December 2015, the daily accumulated precipitation amounted to 115 and 94.2 mm, respectively (Figure 2c).…”

“…Climatically, precipitation is well distributed throughout the year in the study region [35,38], varying from 83.9 mm (April) to 157.4 mm (August). On 22-23 December 2015, the daily accumulated precipitation amounted to 115 and 94.2 mm, respectively (Figure 2c).…”