Abstract
Advances in
space-based observations have provided the capacity to develop regional- to
global-scale estimates of evaporation, offering insights into this key
component of the hydrological cycle. However, the evaluation of large-scale
evaporation retrievals is not a straightforward task. While a number of
studies have intercompared a range of these evaporation products by examining
the variance amongst them, or by comparison of pixel-scale retrievals against
ground-based observations, there is a need to explore more appropriate
techniques to comprehensively evaluate remote-sensing-based estimates. One
possible approach is to establish the level of product agreement between
related hydrological components: for instance, how well do evaporation
patterns and response match with precipitation or water storage changes? To
assess the suitability of this consistency
-based approach for evaluating
evaporation products, we focused our investigation on four globally
distributed basins in arid and semi-arid environments, comprising the
Colorado River basin, Niger River basin, Aral Sea basin, and Lake Eyre basin.
In an effort to assess retrieval quality, three satellite-based global
evaporation products based on different methodologies and input data,
including CSIRO-PML, the MODIS Global Evapotranspiration product (MOD16), and
Global Land Evaporation: the Amsterdam Methodology (GLEAM), were evaluated
against rainfall data from the Global Precipitation Climatology Project
(GPCP) along with Gravity Recovery and Climate Experiment (GRACE) water storage anomalies. To ensure a fair
comparison, we evaluated consistency using a degree correlation approach
after transforming both evaporation and precipitation data into spherical
harmonics. Overall we found no persistent hydrological consistency in these
dryland environments. Indeed, the degree correlation showed oscillating
values between periods of low and high water storage changes, with a phase
difference of about 2–3 months. Interestingly, after imposing a simple lag
in GRACE data to account for delayed surface runoff or baseflow components,
an improved match in terms of degree correlation was observed in the Niger
River basin. Significant improvements to the degree correlations (from
∼ 0 to about 0.6) were also found in the Colorado River basin for both
the CSIRO-PML and GLEAM products, while MOD16 showed only half of that
improvement. In other basins, the variability in the temporal pattern of
degree correlations remained considerable and hindered any clear
differentiation between the evaporation products. Even so, it was found that
a constant lag of 2 months provided a better fit compared to other
alternatives, including a zero lag. From a product assessment perspective, no
significant or persistent advantage could be discerned across any of the
three evaporation products in terms of a sustained hydrological consistency
with precipitation and water storage anomaly data. As a result, our analysis
has implications in terms of the confidence that can be placed in independent
retrievals of the hydrological cycle, raises questions on inter-product
quality, and highlights the need for additional techniques to evaluate
large-scale products.
Citation
ID:
208269
Ref Key:
lpez2017hydrologyevaluating