Rice planting potential in plain-hill-wetland area estimated by nature-society water cycle model

Abstract: With the increasing of well irrigation for rice since the middle of 20 Century, planting area of plain-hill-wetland region has expanded rapidly over its planting potential. And this phenomenon has caused a series of problems, such as the shrinking of wetland area, the decline of groundwater level and the deterioration of water quality. In order to prevent these problems, a two element water cycle model should be constructed to reveal the water cycle mechanism of plain-hill-wetland area under the influence of human activities, and to calculate the suitable area of rice planting. A grid-based distributed hydrological model was constructed by numerical method. The absolute value of relative error between fitted and measured flood peak discharge was in the range of 1.71%-11.55%. The determination coefficient of flood process line was in the range of 0.67-0.85. It indicated that the model could well simulate the hydrological cycle process in the study area. And then a two element water cycle model was further constructed by embedding the influence of human activities. According to the threshold of rice planting potential, parameters such as the maximum groundwater depth and the maximum groundwater level drawdown were extracted. And 18 kinds of scenarios were constructed by different combinations of the parameters and crop species in dry land. The planting potential of paddy field under various scenarios was calculated using the two element model under the condition of satisfying the control threshold, according to mean annual rainfall, evaporation and scenarios. It is observed that the maximum groundwater depth was 5.17-7.49 m, the maximum groundwater level drawdown was 1.67-3.73 m, the maximum slope of the paddy field was in the range of 0.028-0.053, the river water division flow was 50%-70%, the ratio of groundwater extraction was 79%-112%, the rice planting potential was 2.836×105 hm2-5.412×105 hm2, the area from the dry land was 2.105×105 hm2- 4.032×105 hm2, and the area from the unused land was 5.68×104-1.109×105 hm2 for the 18 kinds of scenarios. Taking scenario 17 as an example, the river ecological water demand, groundwater depth and evaporation of dry land crops were tested during the growth stage of paddy fields. In the whole rice growing season, the groundwater depth was less than 7.12 m, and the minimum value of base flow ratio in the developed paddy field was 33.45%. The ratio of mean evaporation from dry land of subdivisions in scenario 17 to that under the same land use scenario in 1990 was in the range of 0.98-1.05. It indicated that the effect of scenario 17 on the river water and groundwater level in the rice growth period was within the control range, while the effect on the dry field was relatively small. Hence, the scenario 17 was the optimal one for application. Currently, the parameters such as hydrometeorology, hydrogeology, irrigation system and groundwater depth were not available. However, the water cycle model constructed in the present study could provide valuable information for describing the hydrological cycle process in plain-hill-wetland area and seeking the potential of paddy field development, based on the full use of the limited parameters.