Introduction
It is widely recognized that hydrological processes and water balances
between mountain and lowland watersheds are significantly different.
Mountain watersheds with sharp wet-dry seasonal transitions and steep
gradients in temperature and precipitation with elevation make the
difficulties lie in quantifying the hydrologic change by integrating the
temporal and spatial distribution of water resources
(Bales et al., 2006;
de Jong et al., 2005). Previous studies
simulated this relatively natural regions based on the supply
(precipitation)-demand (potential evapotranspiration)-storage (water
storage in the soil) hypothesis (Milly,
1994). However, their adjacent downstream areas where slope are flat or
gentle (≤25) have dissimilar hydrological processes relative to
upstream. Streamflow shows crossed, looped and human intervening,
needing another hydrological regime to explain. Few studies have been
conducted to quantify these differences of hydrological characteristics
between mountain and lowland watersheds
(Berihun et al., 2019;
Weingartner et al., 2007).
Climate and landscape characteristics are the primary factors
determining watershed hydrological processes
(Berihun et al., 2019). Their influences
vary with different watershed characteristics and the
agro-ecological settings. For
example, 1) climate and farming controls water sources.Precipitation is the main source for runoff generation in mountain
watersheds, whereas irrigation is another important water source for
lowland watersheds (e.g. accounting for 27% in Lake Taihu Basin, China
(Huang et al., 2018a). 2) Slope is
not the driving force for streamflow. In lowland watersheds, the target
water levels human required regulate the flow direction. 3)Alternative combination of land use options generates huge
discrepancy in hydrological processes . Polder is the main geographic
unit of lowland watersheds. More than half of the total area are covered
by farmland and surface water (Vermaat &
Hellmann, 2010). The
water
storages such as ponds can increase the retention time of surface runoff
by 6 months within polders (Cui et al.,
2019). Moreover, large area of paddy lands will produce
irrigation-related evaporation enhancement. 4) Water conservancy
facilities hinder the natural flow exchange . Pumping stations commonly
existed in lowlands interfere with the hydrological connections between
polders and their receiving water body
(Hesse et al., 2008), for example,
inducing 8.6% reduction of annual runoff to surrounding rivers
(Yan et al., 2018). In conclusions,
predicting the responses of hydrological processes under future climate
change and land use scenarios is favorable for applying sustainable land
and water managements to adapt these impacts, in return resulting to an
increase in available water. However,
the research of individual and
synergetic influence of climate change and land use on hydrological
processes with various watershed characteristics is still limited
(Gusarov, 2019).
The process-based models were widely employed to predict contributions
of driving factors on streamflow variation via physical process
simulations (Chen et al., 2019;
Li et al., 2015;
Wang et al., 2019). However,
most
models (e.g. SWAT, HSPF, Xin’anjiang and MIKE-SHE) were suitable for
freely draining areas with sloping surfaces, and not designed for
lowland polders with shallow groundwater
(Hesse et al., 2008;
Yan et al., 2016). They failed to reflect
the complicated water management operation, especially lacking the
irrigation and drainage processes simulation in crop fields
(Salmon et al., 2015). For example, water
conservancy facilities like dikes dramatically influence in simulating
the quick return flow process through drainage systems
(Tsuchiya et al., 2018). Moreover,
pumping stations result in multiple drainage outlets within polders.
More importantly, many existing models fail to accurate consider the
characteristics of paddy field and ponds. For example, Soil Water
Atmosphere Plant Model (SWAP) treats paddy fields as upland areas, which
actually are the depression areas (Utset
et al., 2007). Others set ponds and paddy lands as reservoirs
(Xie & Cui, 2011). However, water
balance of reservoir is calculated in volume, while paddy fields
calculates water balance in depth
(Tsuchiya et al., 2018). Up to now, the
simulation methods aimed to modeling the hydrological processes in
lowland artificial watersheds are the simplifications of real
situations, inducing that not all the vital processes are taken into
account (Lai et al., 2016;
Su & Luo, 2019).
In our study area, there are two typical hydrological systems: mountain
watersheds and lowland artificial watersheds. Aimed to identify the
relative contributions of climatic and land use to streamflow varying
with different watershed characteristics, we used the developed
raster-based Xin’anjiang model to clarify the mechanism of runoff
generation within mountain areas, and polder areas were treated
separately which used Nitrogen Dynamic Polder (NDP) model to simulate
the artificial drainage and natural flow components
(Huang et al., 2018a). As changes in land
use and climate are expected to intensify in future, it is a task and
challenge to identify further hydrological responses considering these
changes, for devising sustainable land and water management strategies.
The modelling approach in this study is transferable to other
watersheds.