Research On The Effects Of Soil Surface Mulch On Soil Evaporation Using Heat Pulse Method

Soil evaporation is the part of the energy balance and the part of the water balance, so it isan important composition of the vadose zone water and energy research. Land surface coverhas an significant effect on soil evaporation, which can effectively adjust water, fertilizer, gasand heat of soil. It is very important to mulch on the soil surface for water retention. Heatpulse method could be used to measuring soil density, water and the dynamic process of soilevaporation. In this study, the heat-pulse methodology with the11-needle probes was used tomeasuring soil temperature and soil parameters (thermal diffusion, thermal capacity andthermal conduction) of different depth based on the principle of sensible heat balance.Meantime, the short-term coverage (straw, gravel) was used. The objective was to explore thedynamic process of soil evaporation by analyzing the dynamic process of soil temperature,water content and thermal properties and the relationship between soil thermal parametersand soil moisture under bare and mulch conditions.Soil thermal parameters and temperature could be measured with heat pulse technology,and then sensible heat flux, difference in sensible heat flux between the upper and lowerlayers, the latent heat of vaporization, the change in sensible heat storage and soil evaporationrate could be obtained. The measurement results of three depths (6mm,18mm and30mm)showed that soil thermal parameters increased when precipitation happened and decreasedafter precipitation. The deeper soil depth was the more stable the change of soil thermalparameters was. The bare and mulch soil had the same trend of changing. Soil thermalparameters under mulch condition were relatively larger than that under bare conditionbecause soil evaporation could be suppressed after covering stones and straw. Influenced byprecipitation, soil moisture of the near-surface also increased firstly and then decreased withthe drying of soil. There were same results under both bare and mulch, which representedthermal conductivity showed increasing trend of exponential function, a linear relationshipwith the soil heat capacity, and soil thermal diffusion firstly increased and then decreasedwith increasing water content. Besides, the reduction of thermal diffusion showed the apparent hysteresis and the lag time of each depth was more than48hours.The result of the0.5~3mm、3mm~9mm、15~21mm and27~33mm soil payers showedthat mulch on soil surface could reduce soil evaporation and he gravel-mulched andgravel-mulched soil would have delayed peaks of soil evaporation rate, which was ofimportant significance to soil water conservation. Results also showed that the sensible heatflux and the difference in sensible heat flux between the upper layer and lower layer waslower in soil under gravel and straw mulch than in bare soil. As compared with the latter, theformer did not show much change in sensible heat storage in the near-surface soil layer; thesensible heat flux decreased; the difference between the upper and lower parts of a soil layerin sensible heat flux narrowed; the latent heat of vaporization increased. As the energy usedfor soil evaporation decreased, soil evaporation rate also declined. After precipitation,compared with bare soil, soil sensible heat flux and soil temperature gradient (there wasopposite direction of soil sensible heat flux at night and in the daytime; there was oppositedirection of soil temperature gradient at night and in the daytime) under both gravel and strawmulch reduced. The effect of mulch on the upper layer was greater. The p value of the t testbetween the bare and gravel mulch soil of the0.5~3mm,3～9mm and15～21mm soil layerswere0<0.05and of the27～33mm soil layer was0.100>0.05. The p value of the t testbetween the bare and straw mulch soil of the0.5~3mm,3～9mm and15～21mm soil layerswere both0<0.05and of the27～33mm soil layer was0.151>0.05.The soil evaporation ratechanged in the same way as the soil sensible heat flux did. Compared with bare soil, thegravel-mulched soil would have delayed peaks of soil evaporation rate and differencebetween the upper and lower parts of the soil layer in sensible heat flux. In the3～9mm and15～21mm soil layers, the peaks would appear2days later than that in the bare soil. Soilevaporation rate changes trend and the difference of soil sensible heat flux change trend weresimilar. Compared with the bare soil, gravel cover and straw mulch peak evaporation rate hada time lag. The time lag of the present of both0.5~9mm and15~21mm were greater than48hours.From these results, we could conclude that the surface mulch had a significant impact onthe dynamic changes of soil evaporation, which represented that soil evaporation decreasedand there was a time lag of presenting of soil evaporation rate peak. The reason of thereduction of soil evaporation was that the difference in sensible heat flux between the upperlayer and lower layer under mulch condition reduced and the latent heat of vaporization undermulch condition increased, which was relate to soil temperature gradient and soil heatparameters. Soil temperature gradient reduced under mulch condition compared with thatunder bare condition. Soil heat parameters increased under mulch condition compared with that under bare condition.