Watershed water circle dynamics during long term farmland conversion in freeze-thawing area

- Agricultural development increased watershed averaged surface runoff volume 60 mm.
- Expanding paddy land decreased groundwater level at 0.17 m/yr over 15 years.
- Impact of the extreme size of paddy land on percolation was identified.
- Field monitoring and modeling both quantified water recycle dynamics in freeze-thawing area.

SummaryWater resource is increasingly scarce in agricultural watershed under the pressure of socio-economic development. Long term land use conversion and freeze-thawing process posed additional characteristics to the water cycle. The semi-distributed hydrologic model Soil and Water Assessment Tool (SWAT) was employed for surface runoff, evaporation, and percolation simulations in freeze-thawing agricultural watershed. The interpreted five terms of land use data over three decades demonstrated that the percentage of the farmland area of the whole watershed increased from 23.5% to 62.1% and about half of dryland shifted to the paddy land in the recent ten years. The validated SWAT simulation showed that the spatial distribution of the surface runoff volume and the watershed averaged value increased 60 mm. The correlations of precipitation with surface runoff at monthly and yearly scales decreased from 0.8-0.9 to 0.6-0.7 respectively, which highlighted the impact of land use change over the surface runoff. The watershed evaporation was lower under the freeze-thawing condition, which increased from 363.7 mm to 418.5 mm over three decades. The field monitoring recorded the decreasing groundwater level, which was coincided with the expanding area of the paddy land. The watershed precipitation did not varied intensively in the whole simulation period (CV ⩽ 0.01), but the percolation varied as the result of the cultivation disturbance on soil properties. The analysis showed that the expanding paddy land decreased the groundwater level at 0.17 m/yr during 1997 and 2012, which posed new challenge on regional water management. The evapotranspiration in the extreme size of paddy land was relatively small and the groundwater level also decreased relatively slow. These characteristics demonstrated the impact of freeze-thawing on the water cycle. The proposed method can be used as an effective tool for quantitative prediction of irrigation water amount and identify the impact of land use change on the water cycle at freeze-thawing agricultural watershed.