Effects Of Changes In Precipitation On Energy And Water Balances In Meadow Steppes

Under rapid land use and land cover changes and global warming,there is an increasing risk of global extreme climate events with large spatial variations including frequent extreme precipitation events,more intensive and prolonged drought.Using a four-year dataset of two eddy covariance towers,coupled with observations of biomass,phenology and meteorological driving forces,the energy and water exchanges for the meadow steppe in northeastern China and the northern Mongolian Plateau were investigated.Results showed that:(1)Characteristics of environmental factors.The air temperature,soil temperature profiles,vapor pressure deficit,photosynthetically active radiation,annual precipitation and above-ground biomass and leaf area index in the Mongolian Plateau meadow steppe are generally lower than that in the Songnen meadow steppe.The soil water content(SWC)in the Mongolian Plateau meadow steppe showed a stable trend compared with the Songnen meadow steppe during the study period.(2)Characteristics of water balance variation.Precipitation(P)and evapotranspiration(ET)appeared to be more or less balanced in the wet year on the Songnen meadow steppe.The meadow steppe in our study showed a higher cumulative ET than cumulative P in dry years.Although cumulative P was lower than ET at annual scale,the monthly P could sometimes be higher than ET in the meadow steppe during the growing season.(3)Characteristics of energy fluxes variation.Latent heat(LE)dominated the energy exchange of the meadow grasslands on a yearly basis.Throughout the spring drought period in 2007,LE remained low until late summer.In contrast,LE in 2010 remained relatively high from the beginning of the growing season.The Songnen meadow steppe was LE dominated when SWC was >0.3 m3m-3 after intensive precipitation,but switched to H dominated status when soil water content fell below 0.3 m3m-3.In the Songnen meadow steppe,transient waterlogging occurs usually after intensive precipitation.Six transient waterlogging events were detected during the study period.LE increased at the beginning of growing season and decreased after waterlogging at the growing season peak.Based on structural equation modeling(SEM)and characteristics of water-energy balance variation under different precipitation conditions,the study revealed the regulation mechanism of precipitation changes on ecosystem structure and function in meadow steppes.And it can provide a theoretical basis for meadow steppe in response to changes in climate.The conclusions showed that:(1)Analysis of environmental factors.The difference in energy distribution between dry and wet years was mediated by air temperature,soil temperature and soil water content in the Songnen meadow steppe.Soil temperature and vapor pressure deficit had greater impact on LE in dry years,while air temperature,soil temperature and precipitation had a tighter relation with sensible heat(H)in dry years on the Mongolian Plateau meadow steppe.(2)Analysis of water balance.P and ET are the main input and output pathways for the entire ecosystem's water balance.ET>P suggested there were other sources of water to enrich the soil.ET<P suggested a proportion of precipitation infiltrated to recharge groundwater.(3)Effects of changes in precipitation on water-energy balance.Intensive precipitation had a profound impact on water-energy balance that could reduce the damages of drought by elevating deep soil moisture.LE was apparently influenced by the process of temporary waterlogging caused by precipitation >30 mm in a day.The influence of LE on waterlogging depended on timing.LE increased because of low vegetation cover and high groundwater level at the beginning of the growing season.However,LE declined at the growing season peak,which could possibly be attributed to stomatal closure and reduction in both leaf photosynthesis and root respiration.(4)Influence of drought on energy balance.Spring and summer droughts resulted in different energy partitioning between latent and sensible heat energies,with spring drought dramatically decreased the LE fraction due to the change in water.In contrast,summer drought had little impact on LE due to the sufficient water input from large precipitation events at the beginning of the growing season.Overall,sensitive meadow steppes are susceptible to the imbalances in ecosystem structure and function due to the impact of climate changes and human activities.The intensity and frequency of precipitation have changed or are changing in most areas.It is found that single or seasonal precipitation dynamics have a greater impact on plants than annual total precipitation,and have far-reaching implications for ecosystem water-energy balance.Therefore,based on the changes in global precipitation patterns,research on the role of precipitation events is particularly important.