Influence of wetland reclamation on land-surface energy exchange and evapotranspiration in the Sanjiang plain, Northeast China

Abstract

Sustained, large-scale wetland reclamation of paddy fields has been implemented since the 1960s in the Sanjiang Plain, northeast China. The investigation of the effect of this reclamation on land-surface energy exchange and the local water cycle is important for evaluating the stability of regional water resources for future agriculture development and ecological conservation. This study used the eddy covariance method to measure evapotranspiration (ET) and dominant energy transport fluxes for two sites, a typical natural marsh and a paddy field, during the growing seasons of 2017–2018. The two sites showed similar total available energy, whereas the ratio of the latent to net radiative energy (LE/Rn) was substantially lower in the marsh (0.49) compared to the paddy field (0.81), indicating a considerable increase in ET energy consumption after wetland reclamation. The average ET rates of the marsh and paddy field were 2.3 mm d−1 and 3.6 mm d−1, respectively, throughout the growing season. Both the high Priestley-Taylor coefficient (αeq, 1.21 ± 0.20) and the decoupling factor (Ω) suggested Rn-limited conditions in the paddy field, whereas the controlling factor in the marsh changed from vapor pressure deficit (VPD) to radiation energy over the season. The greatest difference in ET occurred in spring due to the alteration in the hydrological environment, including a lowered standing water table and elevated water temperature in the paddy site. The differences in physiology and canopy shape between the paddy and marsh plants contributed largely to enhanced ET during the summer months. In total, alterations in the hydrological environment, canopy structure and plant physiology contributed to the increase in ET, suggesting a synthetic influence of marsh reclamation on surface energy and ET processes. Wetland reclamation increased water consumption through ET and altered the seasonal pattern of the energy balance. This process will inevitably result in increased deficits in regional water resources in the Sanjiang Plain.