Hydrologic and water quality models are often used in assessing the
response of environmental processes to human activities and climatic change.
However, these models differ in terms of their complexity, requirements,
underlying equations, and assumptions, and as such their performance in
simulating landscape processes varies. Consequently, a key question that has to
be addressed is to select the most suitable model that gives results closest to
reality for an intended purpose. In this study, the performance of the
basin-wide older version of SWAT is compared with that of the small watershed
model APEX to assess the performance of both models at a field scale level. The
new restructured version of SWAT (SWAT+) is compared with the older version to
determine whether the new changes incorporated in SWAT+ have improved model
performance, particularly for small watersheds. The three models were used to
simulate the edge of field processes for a 6.6 ha plot located at the
USDA-Agricultural Research Station near Riesel, Texas, and to predict water
yield, soil, and mineral phosphorous loss from the micro watershed. Results
showed that all the uncalibrated models over-predict soil and phosphorous loss
in a micro watershed. Uncalibrated SWAT and SWAT+ models simulated water yield
satisfactory albeit with low-performance metrics. The calibrated versions
simulated water yield with indices close to optimal values. PBIAS as a
performance assessment metric was determined to be overly sensitive and prone
to numerical errors. SWAT+ will be helpful in the understanding of hydrological
and water quality processes at micro watersheds considering that it addresses structural
flaws associated with the older version, and the manually calibrated version
matches the performance of both APEX and SWAT, despite the latter two
undergoing rigorous automatic calibration.
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