Dry Season Evapotranspiration Dynamics over Human-Impacted Landscapes in the Southern Amazon Using the Landsat-Based METRIC Model

Kul Khand,Izaya Numata,Jeppe Kjaersgaard,George Vourlitis

doi:10.3390/rs9070706

Abstract

Although seasonal and temporal variations in evapotranspiration (ET) in Amazonia have been studied based upon flux-tower data and coarse resolution satellite-based models, ET dynamics over human-impacted landscapes are highly uncertain in this region. In this study, we estimate ET rates from critical land cover types over highly fragmented landscapes in the southern Amazon and characterize the ET dynamics during the dry season using the METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) model. METRIC, a Landsat-based ET model, that generates spatially continuous ET estimates at a 30 m spatial resolution widely used for agricultural applications, was adapted to the southern Amazon by using the NDVI indexed reference ET fraction (ETrF) approach. Compared to flux tower-based ET rates, this approach showed an improved performance on the forest ET estimation over the standard METRIC approach, with R2 = 0.73 from R2 = 0.70 and RMSE reduced from 0.77 mm/day to 0.35 mm/day. We used this approach integrated into the METRIC procedure to estimate ET rates from primary, regenerated, and degraded forests and pasture in Acre, Rondônia, and Mato Grosso, all located in the southern Amazon, during the dry season in 2009. The lowest ET rates occurred in Mato Grosso, the driest region. Acre and Rondônia, both located in the southwestern Amazon, had similar ET rates for all land cover types. Dry season ET rates between primary forest and regenerated forest were similar (p > 0.05) in all sites, ranging between 2.5 and 3.4 mm/day for both forest cover types in the three sites. ET rates from degraded forest in Mato Grosso were significantly lower (p < 0.05) compared to the other forest cover types, with a value of 2.03 mm/day on average. Pasture showed the lowest ET rates during the dry season at all study sites, with the dry season average ET varying from 1.7 mm/day in Mato Grosso to 2.8 mm/day in Acre.

Highlights

The Amazon Basin holds nearly half of all remaining tropical rainforest [1,2], playing an important role in regional hydrological cycles
This study addressed two main objectives: (1) adaptation of METRIC to the Amazon region; and (2) characterization of fine spatial scale, dry season ET dynamics from different land cover types in fragmented landscapes in the southern Amazon
0.75 and indicated an overall decreasing trend of pasture greenness as the dry season proceeded at analysis of variance (ANOVA) and student’s t-test at the 5% significance level

Summary

Introduction

The Amazon Basin holds nearly half of all remaining tropical rainforest [1,2], playing an important role in regional hydrological cycles. Tropical forests exchange large amounts of water through evapotranspiration (ET) [3]. Half of all water released to the atmosphere by ET is recycled as precipitation in the Amazon basin [4]. Forest ET in the wet equatorial Amazon is maintained or higher in the dry Remote Sens. Under the longer dry season and more severe water stress conditions, more dynamic and variable ET patterns are found, e.g., higher in the dry season than the wet season and vice-versa, as a function of several factors including vegetation type, depth to the water table, and surface conductance [7,8]

Methods

Results

Discussion

Conclusion