Dynamics Of Soil Reservoir And Evapotranspiration In Dry Farmland On The Loess Tableland,China

Based on long-term field experiments,we investigated dynamics of soil reservoir and evapotranspiration in dry farmland by studying the dynamic change of soil moisture in both winter wheat and spring corn field in the rain-fed Changwu Tableland.Meanwhile,we clarified the water sources for evapotranspiration during the growing period of spring corn and winter wheat,assessed the water balance of the farmland ecosystem,and obtained a preliminary understanding of the formation and recovery process of the dried soil layer,which provided data support and theoretical basis for the production practice and sustainable development of dry farming.The study presented here had the following conclusions:(1)The vertical distribution of annual average soil water content in winter wheat field were "double peaks and double valleys" : first peak and valley occurred in the 10-20 and 50 cm soil layer,respectively,while for the second peak and valley,corresponding soil layer was the 100 and 280 cm soil layer.Soil reservoir did not coincide with precipitation for all yearly precipitations patterns but lagged behind.Annual precipitation patterns had a great influence on the inter-annual and annual dynamics of soil reservoir.Compared with rainy year,the depth of soil moisture consumption decreased in both drought year and normal year and the supplement of precipitation to soil moisture was lower.In rainy year,soil reservoir had a large surplus(84.2 mm);while in normal and drought year,corresponding values were 9.5 and-1.5 mm,respectively.Therefore,we could infer that soil reservoir was compensated in both rainy year and normal year,while in drought year soil reservoir was consumed.In our study,dynamics of soil water in winter wheat field could be divided into four periods: seedling period,slow consumption period,large consumption period,and harvest period and the order of evapotranspiration were large consumption period> seedling period> harvest period> slow consumption period.(2)The vertical distribution of annual average soil water content in spring corn were "single peak and single valley" : peak and valley occurred in the 10-20 and 50 cm soil layer,respectively,and soil water content in 70-100 cm soil layers was relatively stable.Compared with rainy year,root uptake depth decreased in drought year and the soil water content was more stable.The "reservoir level" in both precipitation patterns showed as followings increased firstly,then decreased,and rised again.In drought year,soil reservoir had a large deficit,while for the second and fourth season,corresponding values were 94.3 and 123.7 mm,respectively;while in rainy year,it had a large surplus(208.6 mm).Thus,we could infer that in rainy year water balance was compensated,while in drought year water balance was negatively compensated.The water sources of evapotranspiration during the growing periods differed in both winter wheat and spring corn.For winter wheat,water source of evapotranspiration was consisted of growth period precipitation and fallow period precipitation;while for spring corn,water resource differed in rainy and drought years.Water resource was only precipitation during the growing period in rainy year,while in drought years,water source was consisted of growth period precipitation and soil water storage of last season.(3)Evapotranspiration of farmland could be divided into two parts: actual evapotranspiration during the growing period and invalid evapotranspiration during the dormant period.Growing seasonal mean evapotranspiration of winter wheat was about 540.8 mm,and invalid evapotranspiration accounted for 103.2 mm.For spring corn,these corresponding values were 547.0 and 136.8 mm,respectively.Evapotranspiration of winter wheat and spring corn was the same,and large evapotranspiration losses during the fallow period.The distribution of evapotranspiration during the growing period was single peak in both winter wheat and spring corn field.Winter wheat is more suitable than spring corn for the development of dryland agriculture in rain-fed Changwu Tableland.(4)Yearly precipitations pattern is the dominant factor of dried soil layer formation in winter wheat field,and the uneven distribution of precipitation during the year is the dominant factor in the formation of dried soil layer in spring corn field.The cropping system of winter wheat and spring corn in rain-fed Changwu Tableland will not result in permanent dried soil layer.The process of filling and releasing water in winter wheat soil reservoir presents the characteristics of alternating water filling during large consumption period and slow consumption period,and continuous loss of water during harvest period,fallow period and seeding period.Soil reservoir in the depth of 0-300 cm and 300-600 cm was obviously inconsistent.So if the maximum root depth was used as sampling depth when measuring soil moisture,evapotranspiration during fallow period and seedling period will be overvalued and that during large consumption period and slow consumption period will be underestimated because of negative feedback effect in deep soil reservoir.Transition layer exists in winter wheat field,ranging from 140-360 cm.Time span of crop growth had an effect on the soil reservoir.For example,winter wheat consumed the fallow soil water storage to meet crop needs in the next year,and spring corn utilized the soil water storage to offset crop water shortage that caused by seasonal atmospheric drought or uneven distribution of precipitation.