Research On The Distributed Hydrological Modeling In A Typical Watershed In The Hilly Region Of South China

Hydrological simulation has become an important method for watershed research,management and planning.The characteristics of hydrological processes are different from one watershed to another,influenced by both natural conditions and human activities.It is essential to take the regional characteristics into consideration during hydrological modeling.The hilly region of South China is a typical ecological vulnerable zone in China.It is urgent to use the hydrological modeling approach to facilitate the decision making for sustainable development in this region.The widely-distributed special landscape features(such as paddy fields and ponds)and human activities(such as irrigation and draining)have greatly changed the flow path in natural conditions.The hydrological modeling in this area should consider their effects on the hydrological processes.Although some existing researches of hydrological modeling in the hilly region of South China considered the above mentioned effects,they all used semi-distributed hydrological models,which cannot describe the flow path between the special landscape features in detail.So these researches failed to meet the demands of spatial optimization of management practices in this region.This paper aims to study the fully distributed hydrological modeling methods considering the effects of special landscape features and human activities in this area.The main research contents and conclusions include:1.Construction of watershed routing network,which includes two steps.The first step is spatial discretization.The irregular-polygon-based method was applied for paddy,pond and road,and the raster-based method was applied for others.The second step is the construction of flow tables of the spatial units.We apply different ways in constructing surface and subsurface flow table.The construction of the surface flow table considered the effects of the special landscape features and human activities.The ponds can receive the upslope inflow and irrigate downslope and nearby paddy fields,and their outflow is drained to the downstream ponds or rivers.The paddy fields receive irrigation water from ponds,and their outflow also is drained to the downstream ponds or rivers.The inflow of roads and residents is drained to the downslope pond or river by the drainage network.The forests can receive the upslope inflow,and the outflow is drained to the adjacent downslope spatial units.The subsurface flow path is not effected by the land-surface features,receiving the upslope inflow from the adjacent upslope patches and draining to the adjacent downslope patches.2.Modification of the distributed hydrological modeling methods.Add the sub-processes of effective rainfall and field percolation and define the timing of human activities such as irrigation.The volume of irrigation/draining is calculated according to the difference between the current water depth and the minimum/maximum fitting water depth in different growth periods of rice.The irrigation water of paddy fields comes from the upslope or the nearby ponds.3.Application of the modified model in the ZHONGTIANSHE watershed in Liyang,Jiangsu Province.The main conclusions are as follows.1.The NSE and R2 of the simulated runoff at daily scale using the origin model were 0.50 and 0.63 in the calibration period and 0.62 and 0.61 in the validation period,while those using the modified model were 0.55 and 0.63 in the calibration period and 0.65 and 0.63 in the validation period.The modified model was superior to the former.During rainfall,the simulated runoff of the modified model was lower than that of the origin model,because the irrigation flow made the volume of ponds lower so the pond can intercept more upslope inflow and make the runoff flowing to the outlet lower.2.By analyzing the simulated results of water depth and soil moisture at a typical paddy field and the ponding depth change at a typical pond,the performance of the modified model in simulating the hydrological processes of typical features was analyzed.The results showed that the modified model can properly simulate the water depths change in the rice field during growing periods.The modified model can also simulate the decrease of the depth of ponds during irrigation.3.The simulation units in the model have explicit spatial locations.The ponds can receive upslope inflow and irrigate paddy fields.The modified model can model the water-depth decrease of the pond and the corresponding water-depth increase of the paddy fields during the irrigation activity.Thus it can reflect the interaction among spatial units.In conclusion,the modified model can reflect the effects of the special landscape features and human activities on hydrological processes,and describe the reaction of the spatial units in the hilly region of South China.It can facilitate the hydrological studies and the spatial optimization of the management practices in this region.