Influences Of Additives And Salts On Water Retention And Soil-water Infiltration And Evaporation

The domestic agricultural water comsumption is huge with low use efficiency.Meanwhile,we are facing with shortage fresh-water resource and inconsistency of water supply and demand.Thus,developing poor-quality irrigation helps to alleviate the over-exploitation of freshwater resources.Besides,under the adverse impacts of improper land use patterns,soil stracture is damaged,decreasing water retention.And this requires a method,with low cost and simple operation,which would help to improve water retention.Therefore,the researches into development of poor-quality irrigation and improvement of water retention are of practical relevance,and could reduce agricultural input-to-output ratios.Six types of soils,that is,clay loam soil from Yunnan(YN),sandy loam soil from Liaoning(LN)and Xinjiang(XJ),loam soil from Shandong(SD)and silty clay loam(SX-A)and clay loam(SX-B)from Shaanxi,were selected as experimental samples.Four types of salt solutions with various solutes(i.e.,KCl,NaCl,CaCl2 and MgCl2)and concentrations(i.e.,1,5,30 and 100 g/L)were selected as infiltration water.Three types of soil additives(i.e.,wheat straw,WS;wheat husk,WH;plant dreg,PD)were selected as water-retention materials.An indoor one-dimensional soil column experiment was conducted to make studies on the influences of the type and concentration of salt ions,and the type and application amount of additive on water retention,and soil shrinkage during dehydration;and the influences of salt solutions and additives on infiltration,evaporation,and water distribution.Some results were obtained as follows:(1)The appropriate fitting models for YN,LN,XJ,SD and SX-A were VG-B(m,n),LND-M,VG-M(1-1/n,n),VG-M(1-1/n,n)and VG-B(m,n),respectively.And van Genuchten model could be considered as the optimal model for simulation of SWCCs of the five soils.Besides,the soil shrinkage was ranked in the order that LN>SX-A>YN>XJ>SD,and a logarithmic function was further found between degree of effective soil shrinkage and soil suction during SWCC measurement.Finally,soil shrinkage process could be roughly divided into pseudo-saturated stage,structural stage and super-normal stage.And the three stages could be described by linear function,but the ranges of soil suction corresponding to three soil shrinkage stages were different.(2)WS,WH and PD all could increase soil water retention,ranking in the order that PD > WS > WH,and water-holding capacity was correlated positively with application amount of additives,increasing soil water content by 22.80%,10.93% and 7.19%,respectively.Besides,the modified van-Genuchten model,which is combined with soil capillary theory,had increased applicability to actual field soils mixed with additives.The parameters in the modified model could be calibrated using genetic algorithm.Furthermore,a logarithmic function and a Logistic function were found in the relationship between suction and soil moisture,and soil linear ratio,respectively.Finally,the soil shrinkage was ranked in the order that WH < PD < WS,and the degree of soil crack in the order that PD < WS < WH.(3)The four salt cations(i.e.,K+,Na+,Ca2+ and Mg2+)could weaken soil water retention,and water-holding capacity was correlated negatively with concentration of salt cation.The van Genuchten model was suitable for fitting soil water characteristic curve(SWCC)saturated by saline solution.These four salt cations could also weaken saturated water content,residual water content and air suction,and water-holding capacity was correlated negatively with saturated water content.Besides,K+,a high concentration of Na+ and a low concentration of Mg2+ helped to decrease soil axial shrinkage.And the axial shrinkage strain was correlated negatively with K+(P<0.01),Na+(P<0.05)and Ca2+ concentrations and positively with Mg2+ concentration.Soils shrank during drying with increasing bulk density,ranking in the order that Mg2+>Ca2+>Na+>K+.Finally,the four salt cations helped to weaken cracking based on the quantitative analysis of crack area density and length density.And these two crack indicators was significantly correlated negatively with K+,Na+ and Ca2+ concentrations and positively with Mg2+ concentration.(4)Both PD and WS could decelerate the wetting front migration rate,and they had inhibiting effects on evaporation.Besides,wetting front migration distance,accumulative infiltration and accumulative evaporation all gradually decreased with increasing application amount of soil additive.Kostiakov and Rose models were available for describing accumulative infiltration and infiltration duration,and accumulative evaporation and evaporation duration,respectively.Furthermore,based on the soil moisture profile after infiltration and evaporation,PD was more effective than WS in enhancing water retention.(5)For the soils in Yangling area,the four salt solutions always accelerated infiltration,and both infiltration rate and accumulative infiltration gradually increased with increasing concentration of salt solutions.Ca2+ and Mg2+ were more easily than K+ and Na+ in wetting front movement.Besides,Kostiakov model was available for describing accumulative infiltration and infiltration duration.After infiltration,the water content in soil profiles decreased with increasing concentration of salt solutions.And the soil water content for the treatments using CaCl2 and KCl solutions was the smallest and largest,respectively.For YN,LN,SD,XJ and SX-B samples,the four types of salt solutions could accelerate infiltration,but had little impacts on accumulative infiltration.Salt solution had the smallest influences on wetting front migration rate for SX-B sample,relative to the control treatment.Besides,the four types of salt solutions could also accelerate surface evaporation,and the very salt solution with the smallest influence on water loss for these five types of soil samples was MgCl2,NaCl,CaCl2,KCl and NaCl,respectively.Furthermore,Rose model was available for describing accumulative evaporation and evaporation duration.Finally,MgCl2,MgCl2,KCl,KCl and NaCl solution had the smallest impacts on soil moisture in vertical profiles for the five selected samples.Salt and additive have different impacts on SWCC and soil-water movement.The present study made analyses on water retentnion and soil-water infiltration-evaporation only from the perspective of soil physics.However,salt ions and soil additives have complicated influences on soil properties,which need further analyses combining with soil chemistry.