Study on Field Water Dynamics in Mu Us Sand Land with Shallow Groundwater Table
： 2018 - 07 - 23
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Abstract & Keywords
Abstract: Background, aim, and scope The vegetation water consumption is one critical part composing the transport processes of water and heat in the Soil-Plant-Atmosphere Continuum (SPAC), especially for the arid areas. For the arid areas, water is not only the main factor restricting plant growth, but also vital for the maintenance of the local ecological environment. The Mu Us Sandy Land (MUSL) in the northwest of China is dominated by arid climate with relatively shallow groundwater table, and the MUSL groundwater variation is closely related to that of soil moisture. Considering the critical role of water in the development of agriculture in MUSL region, it will be of great importance to study the field water dynamics in the Mu Us Sandy Land. Since spring maize is the primary crop in the MUSL region, here we examined the key water transport processes during the growing season of spring maize by combining model simulations with field measurements. Materials and methods In order to quantify field water dynamics in the MUSL region, one typical field at the Hailiutu catchment was selected to conduct this study. We collected in-situ measurements of hydrologic factors including soil water content, soil water potential and groundwater table during the whole growing season of spring maize from May to October (a total of 155 days). Soil water content was measured at the depth of 10-90cm with10-cm-spacing every 5 days using Time Domain Reflectometer (TDR). Three TDRs were set with the distance of 3m in this study. Soil water potential was measured with Watermark (with seven sensors) every 10 minutes at six depths. The MiniDiver was applied to monitor groundwater table every 10 minutes. A Bowen-ratio meteorological station was used to observe meteorological variables including wind speed/direction, net radiation, air temperature/humidity and rainfall. On this basis, we parameterized and validated the Hydrus-1D model, and used it to evaluate the field water dynamics during the growing season of spring maize. Results The soil water content above the depth of 40 cm strongly varied with influences of irrigation and rainfall events, while the soil water content below the depth of 50 cm varied similarly with the groundwater dynamics. The simulation results from Hydrus-1D model indicated that, evapotranspiration (ET) of spring maize during the whole growing period reached 580.32mm, in which 31% were contributed by evaporation, and the ET amount during the jointing, earing and filling periods accounted for 71.8% of the whole ET amount. The dynamics of evaporation and transpiration were found to be significantly related with the variation of leaf area index (LAI), with correlation coefficients of -0.599 and 0.712, respectively. The contribution of groundwater for maize growing was 220.09 mm, which accounted for 37.9% of the total ET amount. The model outputs showed that there were about about 8.5 mm of infiltration under the present irrigation amount (~30 mm). Meanwhile, simulation results also indicated that spring maize would not utilize the groundwater when the average depth of groundwater table fell to 147 cm, and the irrigation amount thus turned to be maximum (432.45 mm). Discussion The field soil water content varied dramatically during our study period. However, the main factors influencing the dynamics of soil water content at different depths were different. The soil water content at shallow depths was mainly affected plant growth and meteorological conditions, while the variation of soil moisture at the deep depths was dominant by groundwater dynamics. The evaporation and transpiration and their relative magnitudes varied along the growing season of spring maize due to influences of LAI, which resulted in that the transpiration peaked during earing period and evaporation peaked during sowing period. Conclusions The field water dynamics of spring maize in the MUSL region were closely related with plant growth, meteorological conditions and groundwater table, and the main factors influencing the dynamics of soil water content at different depths were different. The ET mainly occurred during the jointing, earing and filling periods. In order to reduce the deep infiltration and meet the need of water-saving irrigation, the irrigation amount could be reduced to 72% of the original standard at the present groundwater table. For the shallow ground water table, obvious water transport process existed between ground water and soil water, and ground water contributed to the water consumption of spring maize. However, spring maize would not utilize the groundwater when the average depth of groundwater table fell to 147 cm. Recommendations and perspectives Understanding the field water dynamics of spring maize in the MUSL region is not only essential in providing reference basis to the local farmers to improve irrigation use efficiency, but also informative for the water-saving agriculture and ecological environmental protection in Mu Us Sandy Land.
Keywords: Hydrus-1D; water dynamics; evapotranspiration; groundwater utilization

1   研究区概况与研究方法
1.1   研究区概况和试验介绍

 监测项目Monitoring item 仪器Instrument 测量高度Measuring height /cm 测量频率Measuringfrequency 监测时间(月-日)Monitoring time (month-day) 地下水埋深Groundwater depth MiniDiver/MiniBaro −150/−10 10 min一次Once per 10 minutes 04-30—10-01 土水势Soil water potential Watermark −10、−20、−30、−50、−70、−90 10 min一次Once per 10 minutes 06-01—10-01 土壤含水率Soil water content 时域反射仪(Trime-IPH)Time Domain Reflectometer −10、−20、−30、−40、−50、−60、−70、−80、−90 5 d一次, 降雨灌溉前后加密Once per 5 days, and once before and after rainfall and irrigation, respectively 06-01—10-01 雨量Rainfall 自动雨量计Automatic rain gauge 100 04-30—10-01 灌溉量Irrigation 量水堰Measuring weir 06-1—10-01

Fig.1 Schematic diagram of instruments installation locations in the test site of Mu Us Sandy Land
1.2   Hydrus-1D 模型介绍

Hydrus-1D模型用于模拟计算一维垂直非饱和流和溶质运移，不仅考虑了植物根系吸水和土壤持水能力的滞后效应，还加入了气象模块，适用于各种恒定或非恒定的边界条件（Simunek et al，1998）。

$$\frac{\partial \theta }{\partial t}=\frac{\partial }{\partial z}\left[\left({K}_{\left(\theta \right)}\frac{\partial h}{\partial z}\right)-{K}_{\left(\theta \right)}\right]-S$$ （1）

Van Genuchten 模型的表示方式为：
$${\theta }_{\left(h\right)}={\theta }_{r}+\frac{{\theta }_{s}-{\theta }_{r}}{{\left[1+{\left|\alpha h\right|}^{n}\right]}^{m}}$$h<0 （2）
$${K}_{\left(h\right)}={K}_{s}{S}_{e}^{1/2}{\left[1-{\left(1-{S}_{e}^{1/m}\right)}^{m}\right]}^{2}$$h<0 （3）
$${S}_{e}=\frac{\theta -{\theta }_{r}}{{\theta }_{s}-{\theta }_{r}}$$ （4）

$$S\left(z,t\right)=\alpha \left(h,z\right)\beta \left(z\right){T}_{p}$$ （5）

$${T}_{p}=E{T}_{0}\left(1-{\mathrm{e}}^{-kLAI}\right)$$ （6）

k =－0.055LAI+0.622 （7）
1.3   模型建立与参数反演

$${L}_{r}\left(\stackrel{-}{t}\right)=150\left(1-{e}^{-2.1886\stackrel{-}{t}}\right)$$ （8）

 土壤层深度Soil depth /cm $${\theta }_{r}$$ /(cm3·cm-3) $${\theta }_{r}$$ /(cm3·cm-3) $$\partial$$ /cm-1 n K /(cm·d-1) 0-10 0.054 0.392 0.0838 1.35 939 10-20 0.064 0.37 0.0793 1.38 834.2 20-30 0.056 0.365 0.0786 1.44 875.2 30-50 0.044 0.38 0.0536 1.65 351.4 50-150 0.046 0.44 0.0361 2.54 869.7

Fig.2 The relationship between soil-water characteristic curves and measured values at different depth

1.4   模型检验

$$RMSE=\sqrt{\frac{1}{N}\sum _{i=1}^{N}{\left({s}_{i}-{o}_{i}\right)}^{2}}$$ （9）

$$RE=\frac{{\sum }_{i=1}^{N}{s}_{i}}{\sum _{i=1}^{N}{o}_{i}}-1$$ （10）
Nash效率系数（Nash-sutcliffe efficiency coefficient，NSE
$$NSE=1-\frac{\sum _{i=1}^{N}{\left({s}_{i}-{o}_{i}\right)}^{2}}{\sum _{i=1}^{N}{\left({o}_{i}-\stackrel{-}{o}\right)}^{2}}$$ （11）

Fig.3 Simulation and experimental values of spring corn field

2   结果与讨论
2.1   土壤水分变化特征

Fig.4 Dynamic curves of soil water content, irrigation, precipitation and groundwater depth

2.2   水分通量计算

Fig.5 Hydrological process and model output

 生长阶段/日期Growth stage /Date 播种Sowing4.30-5.12 出苗Seeding5.13-6.11 拔节Jointing6.12-7.17 抽穗Earing7.18-8.7 灌浆Filling8.8-9.8 蜡熟Dough9.9-10.1 全期Whole stage4.30-10.1 天数Days /d 13 30 36 21 32 23 155 灌溉降水入渗Infiltration of irrigation and rainfall /mm -95.77 -6.81 -143.15 -62.47 -26.59 -3.21 -338 蒸散Evapotranspiration /mm 35.42 58.13 143.1 131.53 142.05 70.1 580.32 日蒸散Daily evapotranspiration /(mm·d-) 2.72 1.94 3.97 6.26 4.44 3.05 3.74 日蒸发Daily evaporation /mm 2.72 1.54 1.28 0.75 0.59 0.85 1.17 日蒸腾Daily transpiration /mm 0 0.4 2.7 5.51 3.85 2.2 2.57 蒸发量占蒸散量比例Proportion of evaporation in evapotranspiration /% 100 79 32 12 13 28 31 底面均衡Bottom balanced /mm -45.3 -9.24 17.87 41.77 144.91 70.08 220.09 日底面均衡Daily balanced at bottom /(mm·d-) -3.48 -0.31 0.5 1.99 4.53 3.05 1.42

2.3   地下水利用量与埋深关系

Fig.6 Groundwater uptake of spring coin under different depth

3   结论

（1）毛乌素沙地地下水埋深较浅，地下水与土壤水之间联系紧密。受根系吸水和气象因素的影响，10-40 cm深土层水分变化剧烈，而50 cm深以下土层水分变化受地下水的影响更显著。
（2）玉米全生育期的蒸散总量为580.32 mm，其中蒸发占比为31%。玉米的生长与地下水有密切的联系，地下水对玉米生长的水分贡献量为220.09 mm，占耗水总量的37.9%。
（3）现有的灌水量造成了水分的深层渗漏，为合理利用和保护水资源，在不改变灌溉方式的情况下，可将灌溉量减少为原水量的72%。
（4）地下水位的下降会减少作物对地下水的利用量，并且引起灌溉量的增加，当地下水位下降到埋深为147 cm的时候，玉米生长将不再利用地下水，而此时灌溉量也达到最大为432.45 mm。

Allen R G, Pereira L S, Raes D, et al. 1998. Crop evapotranspiration. Guidelines for computing crop water requirements [J]. Fao Irrigation & Drainage Paper , 56.
Bulletin of Soil and Water Conservation , 36(2): 76 – 81.
Cubera E, Moreno G. 2007. Effect of land-use on soil water dynamic in dehesas of Central–Western Spain [J]. Catena, 71(2): 298 – 308.
Duan L, Huang M, Zhang L. 2016. Differences in hydrological responses for different vegetation types on a steep slope on the Loess Plateau, China [J]. Journal of Hydrology, 537(537):356 – 366.
erosion crisscross on the Chinese Loess Plateau [J].Journal of Earth Environment, 6(3): 188 – 194.]
Feddes R A, Kowalik P J, Zaradny H. 1978. Simulation of field water use and crop yield [M]. John Wiley and Sons, New York, NY.
Genuchten, M. T. V. 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils [J]. Soil Science Society of America Journal, 44(44): 892 – 898.
Nash J E, Sutcliffe J V. 1970 . River flow forecasting
Philip J R. 1966. Plant water relatons: Some physical aspects [J]. Annual Review of Plant Physiology, 17(1):245 – 268.
Ritchie J T. 1972. Model for predicting evaporation from a row crop with incomplete cover [J], Water Resources Research, 8(5): 1204 – 1213.
Simunek J, Sejna M, Genuchten, M. T. V. 1998. The hydrus-1D software package for simulating the one 2-dimensional movement of water, heat, and multiple solutes in variablysaturated media [R]. U. S. Salinity Laboratory.
through conceptual models part I-a discussion of principles [J]. Journal of Hydrology , 10(3): 282－290.
Yang, M., Yanful, E. K. 2002. Water balance during evaporation and drainage in cover soils under different water table conditions [J]. Advances in Environmental Research, 6(4): 505 – 521.
Zhu Y H, Ren L L, Skaggs T H, et al. 2009. Simulation of populus euphratica root uptake of groundwater in an arid woodland of the Ejina Basin, China [J]. Hydrological Processes, 23(17): 2460 – 2469.

BAO Han1*, ZHAN Guobiao1, HOU Lizhu2, SHEN Jiangen3

Foundation Item: National Natural Science Foundation of China (41790443, 31700414); Fundamental Research Fund for the Central University(310821171007, 310821173701)

Journal of Earth Environment