Effects of Different Thicknesses of Gravel Covering on Daily Soil Evaporation

Yue ZHOU，Tiena XlE

1.Experimental Farm of Ningxia University，Yingchuan 750100，China；

2.School of Agriculture，Ningxia University，Yingchuan 750021，China

Effects of Different Thicknesses of Gravel Covering on Daily Soil Evaporation

Yue ZHOU1*，Tiena XlE2

1.Experimental Farm of Ningxia University，Yingchuan 750100，China；

2.School of Agriculture，Ningxia University，Yingchuan 750021，China

［Objective］The research aimed to explore the most suitable gravel cover-ing thickness for selenium sand melon in arid region of central Ningxia.［Method］The natural gravel，which was from Nanshantai Region in Zhongwei City，Ningxia，was acted as test materials to study the effects of different thicknesses of gravel covering on daily evaporation using evaporator overall weighing method.［Result］The daily evaporation capacity order of the gravel covering thickness was as follows：CK＞H1（5 cm）＞H2（8 cm）＞H3（10 cm）＞H4（15 cm）.Meanwhile，with the increase of test days，the difference of cumulative evaporation capacity between H3 （10 cm）and H4（15 cm）decreased gradually.Soil evaporation capacity reduced at the pow-er function with the increase of gravel covering thickness，and the decision coeffi-cient of the fitted curve reached to 0.925 5.［Conclusion］With the increase of gravel covering thickness，evaporation capacity of soil reduced gradually，and the soil water content increased gradually.Gravel covering could effectively reduce the evapora-tion.The thicker of covering，the more obvious inhibition effect on evaporation.The thickness of covering should increase moderately to prevent moisture loss from e-vaporation.Gravel inhibition effect on the evaporation wasn't obvious when the gravel covering thickness reached more than 10 cm.10 cm gravel covering was the most appropriate thickness for local natural condition.The soil evaporation capacity along with the change of gravel covering could be simulated with power function e-quation Y=atb.

Thickness；Gravel covering；Soil evaporation

S oil evaporation is the process that water in soil enters into the air via rise and vaporization from soil surface，and is a special stage of soil water movement process. In arid region，rainfall is less，and evaporation causes loss of a large amount of water，thereby restricting improvement of dry agricultural crop field.Because gravel cover plays sig-nificant roles in improving soil environ-ment，increasing crop yield and raising crop quality，the technology is widely applied in agricultural production field of many countries and regions［1］.A lot of researches show that as a kind of traditional cover technology，gravel cover can significantly decrease soil evaporation.The test aimed to study the impact of local natural gravel on soil evaporation under different cover thicknesses，contrast change process of soil water evaporation under differ-ent thicknesses of gravel cover，and analyze cumulative evaporation of soil under different thicknesses of gravel cover.

Materials and Methods

Tested materials

The tested soil was taken from Yanghuang No.2 Pump Station，Nan-shantai， Letai Village， Yongkang Town，Shapotou District，Zhongwei City of Ningxia.Soil texture was sandy soil.After air drying，soil sample was crushed and screened by 2 mm sieve. Particle size of gravel was about 2.5 cm，and its cover thicknesses were 5，8，10 and 15 cm respectively，whichwere showed by H1-H4.

Test methods

To simulate evaporation，evapo-rator used bucket made by PVC pipe bought from market with the diameter of 20 cm and the height of 50 cm.Par-ticle size of gravel was about 2.5 cm，and its cover thicknesses were 5 cm（H1），8 cm （H2），10 cm （H3）and 15 cm （H4）respectively.Meanwhile，a blank control（CK）was set，and each thickness had three repetitions.In the test，we should guarantee upper sur-face alignment of bucket.At the bot-tom of bucket，there was a layer of spun yarn，and it was clipped into ro-tating screw thread of cover at the bot-tom of bucket.Soil column was put in-to plastic basin with water for 2 days，and reached the saturation status via capillary action of soil.Then，it was set statically for 48 hours.After excess water was discharged，different thick-nesses of gravel were covered on soil surface.At 30 cm of height over each soil column，a 200 W of bulb was set to provide heat for evaporation.Indoor temperature and relative humidity were measured by dry and wet thermometer.Via daily weighing，soil evaporation was measured.The light was turned on for 8 hours （09：00-17：00）every day.During whole test period，we should notice ventilation. Electronic balance with 30 kg of mea-suring range and 1 g of sensitivity was used to weigh.

After soilevaporation started，each variation of soil column mass was soil evaporation mass Esoil（kg）on the day.Esoilwas converted into stan-dard soil evaporation E（mm）：E=Esoil× 1 000×10/（102π）.

Table 1 Daily soil evaporation covered by different thicknesses of gravel

Results and Analysis

Seen from Table 1，the mean of daily soilevaporation covered by gravel was significantly lower than that not covering.It illustrated that gravel cover could effectively improve physi-cal property of soil underlying surface. Gravel can provide extra protection for soil surface，decrease runoff genera-tion，protect soil moisture，prevent soil evaporation and guarantee moisture condition required by plant growth.In arid and semi-arid regions，it plays the roles of improving regional eco-envi-ronment，regulating small climate and increasing utilization rates of water and soilresources to implement gravel cover.The sequence of daily soil evaporation covered by gravel was H1＞H2＞H3＞H4，showing that as gravel cover thickness increased，daily soil evaporation gradually decreased，and soil moisture gradually increased. It was clear that when implementing gravel cover，we should appropriately increase cover thickness to hinder wa-ter loss by evaporation，and promote sustainable development of dry agri-culture.

The coefficient of variation（CV）reflects dispersion degree of random variable （soilwater evaporation）. When CV＞1，variable is strong varia-tion.When 0.1＜CV，variable is moder-ate variation.When CV＜0.1，variable is weak variation.By analyzing CV of daily soil evaporation，it was obtained that CV of daily soil evaporation under different thicknesses of gravel cover and not covering conditions was all more than 0.1，showing as moderate variation.CV difference among treat-ments was not large.

Skewness coefficientillustrates symmetry degree of variable revolving mean，and Kurtosis coefficient illus-trates steepness and flatness degrees of variable distribution.Both Skewness coefficient and Kurtosis coefficient of daily soil evaporation not covered by gravel were higher than gravel cover.It illustrated that symmetry and flatness of daily soil evaporation distribution not covered by gravel were worse than gravel cover，that is，extreme value of soil evaporation had larger impact on the treatment not covered by gravel.

Seen from Table 2，cumulative soil evaporation among CK and differ-ent treatments had significant differ-ence （P＜0.05）.Meanwhile，as gravel cover thickness increased，soil mois-ture evaporation also significantly de-creased.

Seen from Fig.1，temporal change rule of daily soil evaporation between different thicknesses of gravel cover and not covering was basically con-sistent，which was mainly affected by meteorological factors（temperature，humidity and wind velocity）and pre-sented continuous fluctuation change. Change magnitude of soil evaporation covered by gravelwas obviously smaller than that not covered，which sufficiently embodied the effects ofgravel cover layer maintaining soil moisture and preventing soil evapora-tion.

Table 2 Cumulative soil evaporation at different periods under different thicknesses of gravel cover mm

Seen from Fig.2，cumulative evap-oration under different cover thick-nesses was obviously smaller than that of bare soil，that is，gravel cover could effectively inhibit soil evapora-tion and improve utilization efficiency of water.Meanwhile，cumulative evap-oration decreased as gravel cover thickness increased.When gravel thickness was 5 cm，cumulative evap-oration was the maximum.When grav-el thickness was 15 cm，cumulative evaporation wasthe minimum.It was similar with test result of Kemper et al［2-3］.Test results showed that grav-el covercould effectively decrease evaporation.Moreover，the thicker the cover thickness，the more obvious the inhibition effect on evaporation.Mean-while，as test days increased，the dif-ference of cumulative evaporation be-tween H3 and H4 gradually de-creased.It illustrated that when gravel cover thickness reached over 10 cm，inhibition effect of gravel on evapora- tion had small difference.Therefore，10 cm of gravel cover thickness was the most suitable for local natural condition.

To clear temporal change char-acteristics of cumulative evaporation，and provide convenient way for pre-dicting cumulative evaporation under gravel cover，temporal change result of cumulative evaporation under differ-ent thicknesses of gravel cover was fitted.Seen from Table 3，correlation coefficient of linear regression was all more than 0.93，and correlation coeffi-cient of power regression was all more than 0.89，while correlation coefficient of polynomial regression was all more than 0.95.It illustrated that polynomial equation Y=at2+bt could be used to simulate change of cumulative evapo-ration as time，that is，cumulative evap-oration increased at the two power function as time.Here，dependent variable Y was cumulative evapora-tion，and independent variable t was evaporation time，while both a and b were equation coefficients.Seen from Fig.3，total soil evaporation presented power function of decrease as gravel cover thickness increased，and deter-mination coefficient of the fitted curve reached 0.925 5.It illustrated that pow-er function equation Y=atbcould be used to simulate change of total soil evaporation as gravel cover thickness.

Table 3 Fitted results for temporal change of cumulative evaporation under different thicknesses of gravel cover

Conclusion

Using weighing method，temporal change process of soil evaporation under different thicknesses of gravel cover was studied.Research showed that as cover thickness increased，dai-ly soil evaporation gradually de-creased，and soil moisture gradually increased.It illustrated that when im-plementing gravel cover，we should appropriately increase cover thickness to hinder mass loss of water and pro-mote sustainable development of dry farming.Both Skewness coefficient and Kurtosis coefficient of daily soil evaporation not covered by gravel were worse than gravel cover.It illus-trated that the impact of extreme value of soil evaporation on not covering was larger.Cumulative evaporation under different cover thicknesses was all smaller than that of bare land.Mean-while，cumulative evaporation de-creased as gravel cover thickness in-creased.Cumulative evaporation was the maximum when cover thicknesswas 5 cm and was the minimum when cover thickness was 15 cm.However，the difference of cumulative evapora-tion between H3 and H4 gradually de-creased.It illustrated that when gravel cover thickness reached over 10 cm，inhibition effect of gravel on evapora-tion had small difference.Cumulative evaporation under different thickness-es of gravel cover presented two pow-er function of increase as evaporation time，and correlation coefficient was all more than 0.95.Total soil evaporation presented power function of decrease as gravel cover thickness increased，and determination coefficient of the fit-ting curve reached 0.925 5.

［1］WANG TS（王天送），SU HC（蘇賀昌），YANG SW（楊世維）.Water characteris-tics of sandy soil in Lanzhou region（蘭州地區(qū)砂田土壤的水分特征）［J］.Agri-cultural Research in the Arid Areas（干旱地區(qū)農(nóng)業(yè)研究），1991（1）：66-69.

［2］KEMPER WD，NICKS AD，COREY AT. Accumulation of water in soil sunder gravel and sand mulches［J］.Soil Sci-ence Society of America Journal，1994，58（1）：56-63.

［3］WEN MX（溫明霞）.Experimental study on water motion of different soil and stone mixed media（不同土石混合介質(zhì)水分運(yùn)動(dòng)的試驗(yàn)研究）［D］.Yangling：Northwest Agriculture and Forestry Uni-versity（楊凌：西北農(nóng)林科技大學(xué)），2011.

Responsible editor：Ping SONG

Responsible proofreader：Xiaoyan WU

不同厚度礫石覆蓋對(duì)土壤蒸發(fā)的影響

周 約1*，謝鐵娜2（1.寧夏大學(xué)實(shí)驗(yàn)農(nóng)場(chǎng)，寧夏銀川 750100；2.寧夏大學(xué)農(nóng)學(xué)院，寧夏銀川750021）

［目的］為了探求寧夏中部干旱地區(qū)最適宜的硒砂瓜覆砂厚度。［方法］采用蒸發(fā)器整體稱重法，以寧夏中衛(wèi)南山臺(tái)地區(qū)天然砂石為試材，研究了不同厚度礫石覆蓋對(duì)土壤蒸發(fā)的影響。［結(jié)果］礫石覆蓋處理的土壤日蒸發(fā)量的大小順序?yàn)镃K＞H1（5 cm）＞H2（8 cm）＞H3（10 cm）＞H4（15 cm）。同時(shí)，隨著試驗(yàn)天數(shù)的增加，H3（10 cm）和H4（15 cm）處理的累積蒸發(fā)量的差別逐漸減小。土壤總蒸發(fā)量隨碎石覆蓋厚度的增加呈現(xiàn)冪函數(shù)減少的變化趨勢(shì)，擬合曲線的決定系數(shù)達(dá)到0.925 5。［結(jié)論］隨著砂石覆蓋厚度的增加，土壤日蒸發(fā)量逐漸減少，土壤含水量逐漸增加。覆蓋可以有效減小蒸發(fā)，且覆蓋厚度越厚，對(duì)蒸發(fā)的抑制效果越明顯。在實(shí)施礫石覆蓋處理時(shí)，應(yīng)該適量增加覆蓋厚度，阻止大量水分因蒸發(fā)而造成的水分損失。當(dāng)?shù)[石覆蓋厚度達(dá)10 cm以上時(shí)，礫石抑制蒸發(fā)的效果差別不大，因此10 cm的礫石覆蓋厚度最適宜當(dāng)?shù)貐^(qū)域的自然條件。冪函數(shù)方程Y=atb可以用來模擬土壤總蒸發(fā)量隨碎石覆蓋厚度的變化。

厚度；礫石覆蓋；土壤蒸發(fā)

寧夏大學(xué)自然科學(xué)研究基金項(xiàng)目（ZR1208）；寧夏高等學(xué)?？茖W(xué)技術(shù)研究項(xiàng)目（NGY2014065）。

周約（1972-），男，寧夏中衛(wèi)人，講師，碩士，從事草地生態(tài)資源與環(huán)境方面的研究。*通訊作者，E-mail：598738044@qq.com。

2015-06-20

Supported by Natural Science Research Foundation Item of Ningxia University，China（ZR1208）；Science and Technology Research Item of Ningxia Colleges and Universities，China（NGY2014065）.

*Corresponding author.E-mail：598738044@qq.com

Received：June 20，2015 Accepted：August 6，2015

修回日期 2015-08-06