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1 Korean J. Soil Sci. Fert. Vol.50, No.5, pp , 2017 Korean Journal of Soil Science and Fertilizer Article pissn : eissn : Interpreting in situ Soil Water Characteristics Curve under Different Paddy Soil Types Using Undisturbed Lysimeter with Soil Sensor Mijin Seo, Kyunghwa Han 1 *, Heerae Cho 1, Junghun Ok 1, Yongseon Zhang 1, Youngho Seo 2, Kangho Jung 1, Hyubsung Lee 1, and Gisun Kim 1 National Institute of Environmental Research, Goryeong 40103, Korea 1 National Institute of Agricultural Sciences, Wanju 55365, Korea 2 Gangwon Agricultural Research & Extension Services, Chuncheon 24226, Korea *Corresponding author: bearthink@korea.kr A B S T R A C T Received: July 4, 2017 Revised: September 15, 2017 Accepted: September 19, 2017 The soil water characteristics curve (SWCC) represents the relation between soil water potential and soil water content. The shape and range of SWCC according to the relation could vary depending on soil characteristics. The objective of the study was to estimate SWCC depending on soil types and layers and to analyze the trend among them. To accomplish this goal, the unsaturated three soils were considered: silty clay loam, loam, and sandy loam soils. Weighable lysimeters were used for exactly measuring soil water content and soil water potential. Two fitting models, van Genuchten and Campbell, were applied. Two models entirely fitted well the measured SWCC, indicating low RMSE and high R 2 values. However, the large difference between the measured and the estimated was found at the 30 cm layer of the silty clay loam soil, and the gap was wider as soil water potential increased. In addition, the non-linear decrease of soil water content according to the increase of soil water potential tended to be more distinct in the sandy loam soil and at the 10 cm layer than in the silty clay loam soil and at the lower layers. These might be seen due to the various factors such as not only pore size distribution, but also cracks by high clay content and plow pan layers by compaction. This study clearly showed difficulty in the estimation of SWCC by such kind of factors. Keywords: SWCC, Weighable lysimeter, Fitting model, Soil types, Soil layers Parameters obtained from the Campbell and van Genuchten models and goodness-of-fit results. Soil texture Layer Porosity Campbell van Genuchten (cm) (%) Ψ b b RMSE R 2 α n RMSE R 2 Sandy loam Loam Silty clay loam C The Korean Society of Soil Science and Fertilizer. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non- Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

2 Interpreting in situ Soil Water Characteristics Curve under Different Paddy Soil Types Using Undisturbed Lysimeter with Soil Sensor 337 Introduction 포화토양은토양내고상, 액상, 기상삼상중기상에의한영향이제외되며따라서물의이동은압력포텐셜과중력포텐셜에의해지배된다. 반면, 삼상으로이루어져있는불포화토양은메트릭포텐셜과중력포텐셜에의한영향을받으며포화토양과는다른물의이동양상을나타낸다. 토양수분특성곡선 (Soil Water Characteristics Curve, SWCC) 은중력포텐셜에의한영향후에불포화토양에서의흡착과모세관현상에의한토양수분포텐셜과토양수분함량의관계를나타내는곡선으로토양내공극분포에따른물의이동특성을파악할수있다. 토양수분특성곡선을통해토양수분포텐셜에따른토양수분함량을예측또는평가함으로써관개시점, 관개량과같은물절약측면에서의효율적인물관리체계를구축할수있다 (Hur et al., 2006). 또한, 토양투수, 수리전도도, 양분이동등의연구에도토양수분특성곡선은활용된다 (Hillel, 1971; Taylor and Ashcroft, 1972). 이와같은토양내물이동양상의해석은대기-식물-토양을통한연속적물순환시스템에중요한정보를제공할수있으며이에토양수분특성곡선의작성은농업수자원관리에중요한부분이라할수있다. 이런토양수분특성곡선의작성에대해많은연구들은수학적모델을적용한계수추정방법을이용해왔으며모델의적합성을검증하여왔다 (Eom et al., 1995; Hur et al., 2006; Hur et al., 2010; Jabro et al., 2009; Kang et al., 2002; Ki and Kim, 2008; Kim, 2003; Lee et al., 2005; Too et al., 2014). 계수추정방법을통해구명된토양수분특성계수들을이용하는것은시간과돈, 노동력절감측면에서큰장점을가지고있다 (Eom et al., 1995; Hur et al., 2006). 또한, 토양수분측정이어려운상황이거나또는측정이가능할지라도불확실성이높은경우매우유용하게사용될수있다. 이런점에서토양특성이제대로반영된계수들을이용하는것은중요하며, 이것은정확한실측자료의이용을통해서가능하다. 하지만지금까지대부분의연구는실측에대한어려움때문에실내실험을통하거나필드에서측정된소수의자료를활용한경우가대부분이었다 (Jung et al., 2015; Ki and Kim, 2008; Kim, 2003; Park et al., 2010; Shin et al., 2004). 이런경우토양샘플링시시료의교란에대한문제 (Jung et al., 2015) 를배제하기힘들다. 이에반해정밀중량라이시미터 (precision weighable lysimeter) 는비교란상태로채취한토양으로충진된시설로자연상태그대로의토양수분이동양상에대한정보를제공할수있는장점을가지고있다. 이시설은자동화된시스템으로토양수분의변화에대해실시간연속측정이가능하며, 측정오차의최소화로정확한자료를제공할수있다 (Seo et al., 2016). 하지만이러한시설을이용하여토양수분특성곡선을연구한사례는찾아볼수없다. 특히, 논토양의경우매년담수후써레질등으로인하여토양구조의변화가무척심함에도현재까지토양수분과관련된연구가미흡한것이사실이다. 토양수분특성곡선의형태와범위는토성, 공극률, 입도분포, 용적밀도등다양한토양특성에따라달라질수있다 (Fredlund and Xing, 1994; Jung et al., 2015; Song, 2013; Tuller and Or, 2004). 토양수분장력의경우층경계에연속적이지만토양수분함량은불연속적이며 (Park et al., 1994), 또한비슷한토양특성으로이루어진한토양일지라도층위별토양수분특성곡선은달라질수있다. 따라서토성별및층위별토양수분특성곡선의변화를살펴볼필요가있다. 그러나토성과층위모두에대한토양수분특성곡선의연구는현재까지미흡한실정이다. 몇몇연구자들에의해준설토, 화강풍화토, 점성토, 사질토등서로다른특성을가진토양 (Kang et al., 2002; Shin et al., 2004; Song, 2004) 에따른토양수분특성곡선의연구가있었으며, Hur et al. (2010) 은적토한토양에대해 120 cm 층위까지 3개층위로나누어층위별토양수분보유특성을해석한바있다. 하지만실측데이터를기반으로한토성별및층위별에따른토양수분특성곡선의작성및해석에대한연구는보고된바없다.

3 338 Korean Journal of Soil Science and Fertilizer Vol. 50, No. 5, 2017 따라서본연구에서는우리나라논토양에대한토성별및층위별수분보유특성정보를제공하기위해물리학적특성이서론다른토양으로충진된중량라이시미터로부터의토양수분포텐셜과토양수분함량실측자료를이용하여추정모형을적용하고평가하였으며, 토성별및층위별토양수분특성곡선의토양특성에따른영향을비교분석하였다. Materials and Methods 시험토양과데이터측정시험은한국의대표적토양인미사질식양토 (Silty clay loam), 양토 (Loam), 사양토 (Sandy loam) 세가지비교란상태의담수이전논토양을대상으로하였다. 미사질식양토는익산에서채취하였으며부용통으로 mesic family of Typic Endoaqualfs에속하며, 양토와사양토는전주지역에서채취하였고강서통으로 mesic family of Anthraquic Eutrudepts에속한다. 각토양에대한토성별및층위별물리적특성은토양입도분포의경우비중계법 (Gee and Bauder, 1986) 으로, 전용적밀도는코아법 (Blake and Hartge, 1986) 을이용하여 3반복으로분석하였으며, 그결과는 Table 1과같다. 미사질식양토는표토의점토함량이 30% 정도로양토나사양토에비하여높은점토분포를보였으며, 양토와사양토의경우는높은모래와미사의함량을나타내었다. 각토양의용적밀도는표토보다는심토에서대체적으로높은경향을보였다. 토양수분특성곡선을작성하기위한토양수분장력과토양수분함량의측정은표면적 1 m 2, 깊이 1.5 m의정밀중량라이시미터시설을이용하였다 (Fig. 1). 토양수분함량과장력은라이시미터 5개층위 (10, 30, 55, 85, 125 cm) 마다 120 로설치되어있는 UMP-1 (Soil moisture sensor; UGT, Germany) 과 Tensio 160 (Tensiometer; UGT, Germany) 센서를이용하여측정하였다 (Fig. 1). 토양수분센서의값은토양수분함량실측치와보정한값을활용하였다. 토양수분장력과토양수분포텐셜은부의관계이며결과는토양수분포텐셜로표현하였다. 일반적으로토양수분특성곡선은습윤과정에서의토양수분포텐셜과토양수분함량의관계를해석하는데어려움이있어건조과정에서더많이해석되어왔다 (Hillel, 1998). 본연구에서도수분이감소하는기간약 10일동안의토성과층위에따른시간별데이터를연속측정하였으며, 측정된데이터는 data logger (UGTLog; UGT, Germany) 를이용하여자동수집하였다. 토양수분포텐셜과토양수분함량은 85 cm와 125 cm 층위에서변화를거의나타내지않았기때문에 55 cm 층위까지토양수분특성곡선의작성에이용하였다. Table 1. Properties of three different types of study soil. Soil texture (Soil taxonomy) Silty clay loam (Typic Endoaqualfs) Loam (Anthraquic Eutrudepts) Soil depth (cm) Bulk density (Mg m -3 ) Distribution of soil particles (%) Sand Silt Clay Soil texture of each layer ± Silt clay loam ± Silt clay ± Silt clay ± Loam ± Loam ± Loam Sandy loam ± Sandy loam (Anthraquic ± Loam Eutrudepts) ± Loam mean ± standard deviation.

Interpreting in situ Soil Water Characteristics Curve under Different Paddy Soil Types Using Undisturbed Lysimeter with Soil Sensor 339 Fig. 1.

4 Interpreting in situ Soil Water Characteristics Curve under Different Paddy Soil Types Using Undisturbed Lysimeter with Soil Sensor 339 Fig. 1. Weighable lysimeter cross-section (a: weighable lysimeter; b: 10 g accuracy load cells; c: sensors for five measurement levels; d: automatic data logger). 토양수분보유모형라이시미터실측데이터를이용한토성별및층위별토양수분특성곡선은두가지추정모형 (fitting model) 을이용하여작성하였다. 다양한모형중토양수분특성곡선해석에대표적으로많이이용되는 van Genuchten (1980) 모형 (Mishra et al., 1989) 과 Brooks and Corey (1964) 모형을바탕으로한 Campbell (1974) 모형을적용하였다. van Genuchten (1980) 의모형은 Mualem (1976) 의경험식을바탕으로하였으며 Eq. 1과같다. (Eq. 1) 여기서, θ e 는유효수분함량, θ s 는포화수분함량, θ r 은잔여수분함량, θ는현재수분함량, n은공극크기분포와관련된계수 (n > 1), m은잔여수분함량과관련된계수 (1-1/n), Ψ는토양수분메트릭포텐셜을나타낸다. α는공기유입가 (Air-Entry Value, AEV) 의역함수 (α>0) 이며, AEV (α -1 ) 는토양공극으로공기의유입이시작되는경계지점에서의메트릭포텐셜을의미한다. van Genuchten (1980) 모형은토양수분메트릭포텐셜의값에상관없이연속적인토양수분함량의값을안정적으로구할수있다 (Park et al., 1994). Brooks and Corey (1964) 모형을토대로한 Campbell (1974) 모형은 Eq. 2와같으며, b는공극크기분포와관련된계수로실측값에선형함수를맞춤으로써결정되는경험상수이며, Ψ b 는공기유입가를나타낸다. Campbell (1974) 모형은토양수분함량에대해 Eq. 3과같이변환할수있다. (Eq. 2)

5 340 Korean Journal of Soil Science and Fertilizer Vol. 50, No. 5, 2017 (Eq. 3) 각경험식으로부터의계수들은엑셀의해찾기과정을통해얻어졌으며, 실측값과추정값사이의오차가최소가되는값을산정하였다. 모형으로부터의예측신뢰도평가는평균제곱근오차 (Root Mean Square Error, RMSE) 와결정계수 (coefficient of determination, R 2 ) 를이용하였다. Results and Discussion 모형적용결과두모형의적용결과전체적으로 van Genuchten 모형이 Campbell 모형에비하여낮은오차와높은결정계수의경향을보이며실측값의경향을더잘추정하는것으로나타났다 (Table 2). 하지만 Figs. 2~4에서알수있듯이두모형사이의차이는미미하였으며, 모든토성과층위에서실측값과추정값사이에작은오차와높은결정계수를보이며두모형모두높은신뢰성을나타내었다 (Table 2). 세토성중에서는두모형모두미사질식양토에서높은오차와낮은결정계수를보이며실측값의경향을가장잘반영하지못하는것으로나타났으며, 양토와사양토는비슷한결과를보여주긴했지만사양토에서실측값의경향을더잘반영하는것으로나타났다. 층위별로는모든토성에서하위층으로갈수록낮은오차를나타내었지만결정계수또한낮아지는경향을보였다. 특히, 미사질식양토의경우 30 cm 층위에서측정값과실측값사이에큰차이를보이며 (Fig. 2) 가장낮은결정계수를나타냈다. 미사질식양토 Table 2. Parameters obtained from the Campbell and van Genuchten models and goodness-of-fit results. Soil texture Layer Porosity Campbell van Genuchten (cm) (%) Ψ b b RMSE R 2 α n RMSE R 2 Sandy loam Loam Silty clay loam Fig. 2. Soil water characteristics curve of the silty clay loam soil at the 10 cm (a), 30 cm (b), and 55 cm (c) layers.

6 Interpreting in situ Soil Water Characteristics Curve under Different Paddy Soil Types Using Undisturbed Lysimeter with Soil Sensor 341 Fig. 3. Soil water characteristics curve of the loam soil at the 10 cm (a), 30 cm (b), and 55 cm (c) layers. Fig. 4. Soil water characteristics curve of the sandy loam soil at the 10 cm (a), 30 cm (b), and 55 cm (c) layers. 에서수분함량의변동폭이세깊이에서양토와사양토에비해작게나타나는데이는수분포텐셜 -100 kpa에해당하는공극크기인 3 mm보다큰공극량이작다는것을말한다. Han et al. (2008) 에서도점토함량이낮은사양토에서수분함량의변동폭이가장컸으며미사질식양토에비해모형의적합도가높게나타났다. 토양수분특성곡선을결정하는토양수분특성계수인공기유입가 (AEV) 는토성별및층위별물리적특성에따라다르게나타났다 (Table 2). 이계수는다른연구 (Hillel, 1998; Park et al., 2010) 에서와같이큰공극을가진조립질토양인사양토로갈수록감소하고점토의함량이높은미사질식양토로갈수록증가하는경향을나타내었으며, 사양토에서경계지점의공기유입현상을더욱분명하게드러냈다. 하지만 30 cm 층위에서는미사질식양토에서다른토성보다매우낮은 AEV의값을드러냈으며, 55 cm 층위에서는사양토에서다른토성보다매우높은 AEV의값을드러냈다. 각토성의층위별결과는 Hur et al. (2010) 의결과와비슷하게층위가깊어질수록 AEV가증가하는경향을나타냈다. 하지만양토의 30 cm 층위에서는 55 cm 층위보다높은 AEV의값을나타냈다. Assouline (2006) 은토양수분특성곡선은토양의공극률과용적밀도에의해크게좌우됨을언급한바있는데, 본연구에서 30 cm 층위가 55 cm 층위에비해 AEV 값이높은것또한써레질등으로분산된입자가대공극을막아높아진용적밀도에의한영향으로판단된다. 토양수분특성곡선분석전체적으로토성별및층위별토양수분특성곡선은토양수분포텐셜이증가할수록토양수분함량이감소하는경향을나타냈다 (Figs. 2~4). 토양수분포텐셜에대한토양수분함량은모세관현상과공극분포와같은토양구조에크게영향받는다 (Hillel, 1998). 따라서다른토양입자분포를나타내는세토양마다다른형태와범위의토양수분특성곡선을나타냈다 (Fig. 5). 미사질식양토는다른연구 (Fredlund and Xing, 1994; Hillel, 1998; Park et al., 2010) 에서언급된바와비슷하게다른두토성에비하여가장넓은토양수분포텐셜분포와가장높은토양수분보유범위를나타냈으며가장완만한곡선의기울기를나타냈다. 이것은점토함량이높은토양에서공극이작고물과토양입자사이에흡착에의한영향이크게작용하므로더많은물이포획되며, 따라서토양수분포텐셜이

7 342 Korean Journal of Soil Science and Fertilizer Vol. 50, No. 5, 2017 Fig. 5. Comparison of the measured soil water characteristics curves among the three soils at the 10 cm (a), 30 cm (b), and 55 cm (c) layers. 증가함에따라토양수분함량이서서히감소했기때문으로설명될수있다 (Hillel, 1998). 반면, 모래함량이클수록공극이크며흡착에의한영향보다는모세관현상이지배적이게된다. 따라서사양토로갈수록보수력이작아지고토양수분함량의감소에대한기울기가커지는경향을보였다. 이러한토양수분특성곡선의형태는 10 cm 층위에서더넓은토양수분함량과토양수분포텐셜의분포를보이며다른층위에비하여더뚜렷한형태를드러냈다 (Fig. 5). 물이동의공간적변이는하위층으로갈수록그변화범위가더욱좁아지며곡선의형태가뚜렷하게나타나지않는경향을보였다. 이것은토양수분의변화가하위층으로갈수록크지않으며, 여러요인들에의해토양수분이동이영향을받았기때문이다. 우선, 수분보유력을살펴보면각토성은 10 cm 층위에서높은수분함량을, 30 cm와 55 cm 층위에서는유사한경향을나타냈다. 하지만사양토의경우 55 cm 층위에서 30 cm 층위보다높아진수분보유력과매우좁은수분포텐셜의범위를나타냈다 (Fig. 4). 이는 55 cm 층위근처에다짐층의존재로인한물고임현상으로나타난결과로판단되었다. Seo et al. (2016) 도경운에의한쟁기바닥층등의다짐층에의한물이동의제한을언급한바있다. 사양토 55 cm 층위에서다른토성에비해높은 AEV의값을나타낸것또한다짐층에의한영향으로설명될수있을것으로보인다. 미사질식양토 30 cm 층위의경우는토양수분특성곡선의추정값과실측값사이에큰차이를나타냈다. 토양수분포텐셜 -2 ~ -7 kpa에서 2% 정도수분이감소하다가이후변동은있으나감소경향을나타내지않았다 (Fig. 2(b)). 미사질식양토 30 cm 층위는점토함량이 40% 로매우높기때문에담수시팽창된토양이낙수후수축되어균열에의한공극이생성될수있다. 토양수분특성곡선으로볼때 -2 ~ -7 kpa 범위에평균지름 60 mm의대공극이약 2% 정도생성된것으로파악되었다. 이균열은 30 cm 층위에서양토와사양토 (-10 ~ -30 kpa) 보다미사질식양토 (-2 ~ -40 kpa) 가토양수분포텐셜의최저값을더낮게하는데기여한것으로판단된다. 반면 -10 ~ -40 kpa 범위의공극량은 0.5% 이하로낮게나타났다. 즉, 미사질식양토 30 cm 층위는균열에의한대공극 (60 mm) 에서소공극 (10 mm 이하 ) 으로공극크기가급격히달라지는분포를보이고있다. 이런공극분포일때는 van Genuchten과 Campbell 모형과의적합도가낮다고판단할수있었다. 다만 30 cm 층위에서낮은 AEV 값은균열에의한대공극이있음을가리킨다고할수있다. 미사질식양토 55 cm 층위에서는수분포텐셜 -2 ~ -7 kpa 범위의균열에의한대공극은나타나지않았으며 -10 ~ -40 kpa 범위의공극량은 30 cm 층위와유사하게 0.5% 이하로낮게나타났다. 양토와사양토에서 van Genuchten과 Campbell 모형의적합도가높았으나미사질식양토 30 cm 층위와같이공극크기분포가대공극과소공극으로이원적일경우는모형의적합도가낮다고판단할수있었다. 토양수분특성곡선에서 van Genuchten과 Campbell 모형의변수가추정되면이모형을활용하여불포화상태의토양에서물의이동을해석할수있다 (Han et al., 2008).

8 Interpreting in situ Soil Water Characteristics Curve under Different Paddy Soil Types Using Undisturbed Lysimeter with Soil Sensor 343 Conclusions 본연구에서는비교란중량라이시미터의논토양에장착된토양수분센서와토양수분장력계를활용하여토성별및층위별토양수분특성곡선을작성하고 van Genuchten과 Campbell 모형의적합도를파악하고차이를분석하였다. 논은담수와낙수를반복하면서토양의구조특성이달라질수있어본연구와같은비교란상태에서의토양수분특성곡선의도출은유용한자료로활용될수있으며, 농경지현장상태의물질이동특성을유추하는데기여할수있을것으로판단된다. Acknowledgement This study was financially supported by a grant from the research project (No. PJ010867) of National Institute of Agricultural Sciences, Rural Development Administration, Republic of Korea. References Assouline, S Modeling the relationship between soil bulk density and the hydraulic conductivity function. Vadose Zone J. 5(2): Blake, G.R. and K.H. Hartge Bulk density in methods of soil analysis, part1. In A. Klute (ed.). Monograph No. 9, ASA, Madison, Wisconsin, USA. Brooks, R.H. and A.T. Corey Hydraulic properties of porous media, Hydrology Paper 3:27. Colorado State University (Fort Collins), Colorado, USA. Campbell, G.S A simple method for determining unsaturated conductivity from moisture retention data. Soil Sci. 117(6): Eom, K.C., K.C. Song, K.S. Ryu, Y.K. Sonn, and S.E. Lee Model equations to estimate the soil water characteristics curve using scaling factor. Korean J. Soil Sci. Fert. 28(3): Fredlund, D.G. and A. Xing Equations for the soil-water characteristic curve. Can. Geotech. J. 31(4): Gee, G.W. and J.W. Bauder Particle size analysis. In: methods of soil analysis, part1. In A. Klute (ed.). Monograph No. 9, Am. Soc. Agron., Madison, Wisconsin, USA. Han, K.H., H.M. Ro, H.J. Cho, L.Y. Kim, S.W. Hwang, H.R. Cho, and K.C. Song Unsaturated hydraulic conductivity functions of van Genuchten s and Campbell s models tested by one-step outflow method through tempe pressure cell. Korean J. Soil Sci. Fert. 41: Hillel, D Soil and water: physical principles and processes. Academic press, New York, USA. Hillel, D Environmental soil physics: fundamentals, applications, and environmental considerations. Academic Press, California, USA. Hur, S.O., K.H. Moon, K.H. Jung, S.K. Ha, K.C. Song, H.C. Lim, and G.G. Kim Estimation model for simplification and validation of soil water characteristics curve on volcanic ash soil in subtropical area in Korea. Korean J. Soil Sci. Fert. 39(6): Hur, S.O., S.H. Jeon, K.H. Han, H.R. Jo, Y.K. Sonn, S.K. Ha, J.G. Kim, and N.W. Kim Application of analysis models on soil water retention characteristics in anthropogenic soil. Korean J. Soil Sci. Fert. 43(6): Jabro, J.D., R.G. Evans, Y. Kim, and W.M. Iversen Estimating in situ soil-water retention and field water capacity in two contrasting soil textures. Irrig. Sci. 27(3):

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