Discussion
Model performance
Sensitive parameters determined the model performance. Their initial
ranges were referred to Zhao et al.
(2011) and the optimum parameter sets were obtained from the simulation
results with the maximum NS value. Compared with previous case
studies in Xitiaoxi watershed for mountain hydrology modeling by
Chen et al. (2019), the NS value
and R2 value of daily flows at the Hengtangcun
(Fig. 2) implied that the simulation results by Xin’anjiang model were
“very good” during the calibration and validation periods. According
to Moriasi et al. (2007), streamflow
simulations were regarded as satisfactory for SWAT when NS ≥0.5.
In lowland artificial watersheds, the NDP model considered the
dominating mechanisms affecting polder water balance. They were
artificial drainage, the water interactions among surface water and
groundwater, as well as soil water in farmlands
(Huang et al., 2018a). The NSvalues of water level were above 0.5 during the calibration and
validation periods. This model fit was acceptable compared with the
surveying watershed modeling cases using 257 models
(Wellen et al., 2015). Our study area was
close to the case study by Huang et al.
(2018a) in space and under the unified polder-controlled strategy by
Taihu Basin Authority of Ministry of Water Resources. Therefore, these
similarities between the hydrological conditions in the previously
modelled data and the current hydrological year being modelled can
illustrate the significance and reliability of the model fits.
The different responses of hydrological processes to climate
change and land
use
Our study found that climate and land use change can cause higher
surface runoff, which was consistent with previous study by
Berihun et al. (2019). However, Berihun
did not summarize the differences of hydrological responses between his
three study watersheds with different biophysical characteristics, which
was the key augmentations of our study.
Wang et al. (2019) found that the
corresponding sensitivity of streamflow to changes in climatic
conditions and human activities varying from watershed to watershed.
Therefore, it was important to analyze the hydrological processes and
their driving factors at watershed scales.
In our study, climate change played a more critical role on the
hydrological process in mountain watersheds than in lowland artificial
watersheds. The variation of mean annual discharge resulting from
climate change mainly due to the precipitation factor during the study
period across two watersheds (R2 =0.78 in
mountain, R2 =0.96 in lowland). In mountain
watersheds, the annual average discharge would increase
10~200% from 2015 to 2100 under three climate scenarios
(RCP2.6, RCP4.5 and RCP8.5 from CMIP6), comparing to
10~60% in lowland watersheds. In terms of seasonal
discharge variation, it showed remarkable change during autumn and
winter in mountain watersheds as their small base values. However, rice
seasons were the “hot moments” of flood in lowland artificial
watersheds.
Land use played a more critical role on the hydrological process in
lowland artificial watersheds than in mountain watersheds. Converting
cultivate land to residential land made a significant improvement on
annual average discharge comparing to other land use scenarios, with an
increment of 22.0% in lowland and 7.8% in mountain respectively, under
the same conversion rate of 36.1%. In terms of the “hot moments” of
flood under land use conversion scenarios, in lowland watersheds, the
effect of converting cultivate land to residential land on seasonal
runoff variation was more than 20.0% in the whole year expect winter.
Moreover, when converting cultivate land to water area, seasonal runoff
increased more than 10.0% in rice seasons especially during irrigation
periods. In mountain watersheds, urban expansion from other land use
types such as forestland and grassland caused seasonal runoff increasing
especially in May, during the crops and plants growth seasons when
consuming a lot of water. Another found in mountain watersheds was that
runoff showed a positive correlation with the slope, implicating that
the weak water storage capacity of sloping regions lead to more frequent
rainstorm-runoff processes.
Land use can enhance climatic impact on hydrological process in lowland
artificial watersheds, comparing to the non-significant tendency in
mountain watersheds. For the 2050s, climate change would cause an
increasing annual runoff by 312.9 mm and 349.7 mm in mountain and
lowland watersheds, respectively. When combined with land use change,
annual runoff increased another 55.1 mm and 269.3 mm in mountain and
lowland watersheds, respectively.
Implication for water
managements
As land use and climate changes are expected to be intensive in future,
many regions in the world may suffer from frequent droughts and floods.
The relative effects of climate change and human activity vary among
different watersheds as well as different periods
(Ye et al., 2013). However, the effects
of climate change and land use are always underestimated or
overestimated by different methods, interfering decision makers to
manage water resources in a sustainable way
(Wang et al., 2019).
In our study, increased precipitation and temperature under future
climate conditions would cause higher streamflow especially in mountain
watersheds, which seems to have slight impact on evaporation. Therefore,
floods in sloping regions may become more frequent under future climate
effects, leading to more severe flood disasters. The major effects of
land use change on hydrological processes for the 2050s will happen when
the urban area expanding rapidly at the expense of cropland. Cropland is
an important land use type in lowland artificial watersheds, for the
disappearance of its water retention capacity will destroy flood
prevention by polders. Moreover, pumping stations setting for cropland
can increase the hydraulic retention time in lowland watersheds. The
“hot moments” of flood in lowland artificial watersheds would be rice
reasons when converting cropland to residential area, comparing with
crop growing seasons in mountain watersheds without pumps. To sum up,
precisely forecasting the mountain torrents during rainstorms,
controlling urban expansion and maintaining the cropland area of polders
could be potential strategies for flood prevention and water resources
protection in highland-lowland watersheds.