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  土壤学报  2025, Vol. 62 Issue (4): 998-1009      DOI: 10.11766/trxb202404220165       CSTR: 32215.14.trxb202404220165
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引用本文  

梁静, 王国梁, 徐肖阳, 等. 黄土丘陵区柠条人工林土壤水分动态及其对降水的响应. 土壤学报, 2025, 62(4): 998-1009.
LIANG Jing, WANG Guoliang, XU Xiaoyang, et al. Dynamics of Soil Moisture and Its Response to Rainfall in Caragana korshinskii Plantation in Loess Hilly Region. Acta Pedologica Sinica, 2025, 62(4): 998-1009.

基金项目

国家自然科学基金项目(42041005-3B、42130717)资助

通讯作者Corresponding author

王国梁,E-mail:glwang@nwsuaf.edu.cn

作者简介

梁静(1999—),女,硕士研究生,主要从事流域管理研究。E-mail:liang0914@nwsuaf.edu.cn
黄土丘陵区柠条人工林土壤水分动态及其对降水的响应
梁静1, 王国梁1,2, 徐肖阳1, 刘莹1,2, 冒吉荣1, 曾岩1    
1. 西北农林科技大学水土保持科学与工程学院, 陕西杨陵 712100;
2. 中国科学院水利部水土保持研究所, 陕西杨陵 712100
摘要:土壤水分的季节动态及其沿土层深度变化制约着柠条人工林的生长发育。在黄土丘陵区,降水类型及降雨量如何影响林地土壤水分含量,降水补给是否满足柠条林生长季的用水需求尚不清楚。本文基于2019—2023年纸坊沟流域柠条林野外长期定位监测数据,分析了柠条林不同深度土壤水分的月动态变化,探究了植物生长季节土壤水分对不同降水类型和降雨量的响应过程。结果表明:(1)研究区降水按照降水特征可分为5类降水:小雨、中雨、大雨、暴雨、大暴雨。研究区46%降水事件属于中雨类型,暴雨虽然发生频率较低(19%),但在补给土壤水分等方面发挥着不可替代的作用。(2)土壤水分含量受到降水影响,在年内变化呈现“双峰”型变化。在柠条生长旺盛的6—7月,50~100 cm均出现了季节性干层,土壤干层出现的时间与当年的降水情况密切相关,干层在充足的降水补给后会消失。全年降水量总体上可以满足柠条生长所需。(3)降水类型是影响土壤水分入渗深度和补给量的关键因素。其中暴雨和大雨入渗深度最深,可达100~200 cm;其次是中雨,达10~70 cm;小雨仅局限于0~10 cm。暴雨的土壤水分补给量最大,平均占降水量的87.5%,大雨(36.2%)和长历时中雨(29.7%)次之,而小雨和短历时中雨的降水量则大部分以蒸散发的形式损失。研究发现暴雨和大雨对土壤水分的补给可以缓解50~100 cm季节性干层,而且50 cm以下的土壤在补给后能维持较高的含水状态,为柠条生长提供了有利的水分储备。本研究对黄土丘陵区柠条林可持续管理提供了理论支持。
关键词黄土丘陵区    土壤水分    柠条人工林    降水响应    
Dynamics of Soil Moisture and Its Response to Rainfall in Caragana korshinskii Plantation in Loess Hilly Region
LIANG Jing1, WANG Guoliang1,2, XU Xiaoyang1, LIU Ying1,2, MAO Jirong1, ZENG Yan1    
1. College of Soil and Water Conservation Science and Engineering, Northwest Agriculture and Forestry University, Yangling, Shaanxi 712100, China;
2. Institute of Soil and Water Conservation, Ministry of Water Resources, Chinese Academy of Sciences, Yangling, Shaanxi 712100, China
Abstract: 【Objective】The seasonal dynamics of soil water and its changes along the soil depth restrict the growth and development of Caragana korshinskii plantation. Thus, by exploring the effects of precipitation types and rainfall on soil water content in the loess hilly area, this study seeks to clarify whether precipitation replenishment could meet the water demand of the growing season of Caragana korshinskii forest, and provide theoretical support for the ecological construction of artificial Caragana korshinskii forest in the loess hilly area.【Methods】The analyses in this study are based on precipitation and soil moisture data of Ansai Lime stick forest in Shaanxi Province from 2019 to 2023. The monthly dynamic changes of soil water under Caragana korshinskii forest at different depths were analyzed, and the response process of soil water to different precipitation types and rainfall during the growing season was investigated. The precipitation data was recorded every 30 minutes, and the soil water data were repeated at three monitoring points on the slope and under the slope, with the monitoring frequency of once an hour. The soil volumetric water content at 10 depths (10, 20, 30, 50, 70, 100, 200, 300, 500, 1 000 cm) was monitored.【Results】(1) The precipitation in the study area could be divided into 5 types according to the characteristics of precipitation: light rain, moderate rain, large rain, heavy rain, and very torrential rain. Most of the precipitation events in the study area belonged to moderate rain type, with 59 events accounting for 46% of the total events. (2) Soil moisture content was affected by precipitation, and the annual variation of 0~100 cm soil moisture content showed a "double peak" pattern. In May to August, when Caragana korshinskii was growing vigorously, the seasonal dry layer appeared in the 50~100 cm soil layer. The occurrence time of the dry layer was closely related to the precipitation of the year, and the dry layer disappeared after sufficient precipitation recharge. (3) There was a significant positive correlation between rainfall type and soil water infiltration depth (R2 > 0.81), in which the infiltration depth of heavy rain and heavy rain was the deepest, reaching 100~200 cm, followed by moderate rain 10~70 cm, while light rain was limited to 0~10 cm. The response time of surface and deep soil of heavy rain was the shortest, followed by heavy rain, while the response process of light rain was relatively slow. Also, the soil water supply of rainstorm were significantly higher than that of other rainfall types, accounting for 87.5% of the secondary precipitation, followed by heavy rain (36.2%) and long-duration moderate rain (29.7%), while the precipitation of light rain and short-duration moderate rain was mostly lost in the form of evapotranspiration.【Conclusion】(a) Most precipitation events in the study area belonged to the moderate rain type. The recharge efficiency of light rain and moderate rain was low, and more was loss in the form of evapotranspiration. Although the occurrence frequency of heavy rain was low, it played an irreplaceable role in replenishing soil water. (b) During the flourishing period of Caragana korshinskii growth in summer, the 50~100 cm soil layer generally had different degrees of seasonal drought, which adversely affected the normal growth of vegetation. However, the infiltration depth of heavy rain and rainstorm events reached approximately 50 cm and replenished soil water, and the annual precipitation could meet the needs of Caragana korshinskii growth.
Key words: Loess hilly region    Soil moisture    Caragana plantation    Rainfall response    

黄土丘陵区属于半干旱气候,降水稀缺且分布不均,加之土层深厚、地下水位埋深,土壤水分成为制约当地生态系统过程和功能的关键因素。该地区土壤水分主要依赖天然降水补给,降水的入渗深度决定着该地区的土壤水分状况[1-2]。近年来,黄土高原的植被建设虽有效减少了水土流失、改善了区域生态环境,但也有研究表明,不合理的植被建设导致了土壤水分的过度消耗,从而导致了土壤干燥化,限制区域人工林生态系统健康和稳定发展[3-5]

近年来对土壤水分动态变化过程的研究结果表明:土壤水分含量季节变化与当地的雨季和干季交替密切相关;从垂直分布上看,土壤水分含量具有明显的垂直变异性,受季节变化、根系分布等的影响显著[6-10]。但是上述研究对乔木的土壤水分变化研究的较多,对于柠条等灌木的土壤水分变化研究不足,然而相对于乔木,灌木根系分布较浅且冠幅较小、蒸腾较弱[11-12],用水量不高,灌木林下土壤是否出现季节性干层还未可知,降水对土壤水分的补给是否满足植物的生长水分需求还有待研究。

关于降水对土壤水分的影响,主要包含降水量、降水强度和降水历时的影响。土壤水分与降水量、降水强度和降水历时呈显著正相关,随着深度增加,土壤水分对降水的响应程度减小,降水量较降水强度和降水历时对土壤水分的影响更显著[13-16],但是降水对土壤补给的深度和过程还未明确,经过补给后的土壤水分是否超过植物萎蔫系数,是否能够满足植物生长吸水需求,还未可知。

柠条(Caragana korshinskii)具有耐旱、耐寒、耐瘠薄等特征,合理种植具有水源涵养功能,可以提升生态系统水源涵养质量,在黄土高原丘陵区得到了广泛种植[17-18]。本研究基于2019年3月—2023年12月陕西安塞柠条林野外定位连续监测数据,结合降水数据,对黄土丘陵区典型灌木柠条人工林的自然降水下的土壤水分变化特征及其对降水的响应过程进行分析,旨在探究当前自然降水下的土壤水分是否能满足柠条林生长。为当地柠条林的生态建设提供科学依据,并为黄土高原植被恢复与生态环境改善、区域水资源合理利用提供理论支持。

1 材料与方法 1.1 研究区概况

研究区位于陕西省延安市安塞区纸坊沟小流域(36°42′42″—36°46′28″N,109°13′46″—109°16′03″E),属于温带大陆性气候,年平均气温为8.8 ℃,无霜期约为157 d,年降水量约为550 mm,6月至9月期间降水量约占全年总量的81%。土壤类型以黄绵土为主,土层深厚,质地疏松。流域内现有植被主要为退耕还林(草)工程营造的人工林和封禁后恢复的天然灌丛及草地。其中,乔木主要有刺槐(Robinia pseudoacacia)、油松(Pinus tabuliformis)、侧柏(Platycladus orientalis)等;灌木主要有柠条(Caragana korshinskii)、沙棘(Hippophae rhamnoides)、狼牙刺(Sophora davidii)等;草本主要有白羊草(Bothriochloa ischaemum)、狗尾草(Setaria viridis)、铁杆蒿(Artemisia gmelinii)等。

本试验地为人工柠条林地,样方大小为300 m2(20 m×15 m),位于半阴坡,坡度为25°,海拔1 298 m,退耕年限为25 a,样地草本主要有甘菊(Chrysanthemum lavandulifolium)、黄花蒿(Artemisia annua)、细裂叶莲蒿(Anemone dichotoma)等。

1.2 降水和土壤水分观测方法

本试验采用翻斗式雨量计观测,分辨率为0.2 mm(Campbell Scientific公司,美国)每30 min记录一次降雨量。降水量数据记录于HOBO数据采集器(H07-002-04,Onset Computer Corporation公司,美国)。

本试验采用时域反射仪(TDR),分辨率为0.1%vol(CS655 Campbell Scientific公司,美国),在样地对10个深度(10、20、30、50、70、100、200、300、500、1 000 cm)的土壤容积含水量进行长期定位监测,监测频率为每小时一次。本研究以坡上、坡中和坡下三个监测点作为重复,容积含水量标准差如图 1所示。

注:图中误差线为标准差。  Note:Error bars in the figure indicate standard deviation. 图 1 土壤容积含水量标准差图 Fig. 1 Standard deviation diagram of soil volume water content
1.3 数据处理

降水事件:将间隔超过2天的降水记录的时间定义为单个降水事件的起始时间,考虑到分类过程中极小降水的历时和雨强会影响分类结果的合理性,同时又为全面呈现研究区的降水情况,因此将雨量小于2 mm的降水剔除。

降水-土壤水分响应过程:当至少连续4天无降水记录后发生1 h降水量超过0.4 mm(高于仪器分辨率)的降水时,则认为是一次降水-土壤水分响应过程的开始;当连续4天的累计降水量小于0.2 mm时,则认为是一次降水-土壤水分响应过程的结束。因此,一个降水-土壤水分响应过程可能包含多个降水事件[19-20]

土壤蓄水量和蓄水量变化量计算:

$ \begin{aligned} & W=\mathrm{VWC} \times h \\ & \Delta W=W_2-W_1 \end{aligned} $ (1)

式中,W:土壤蓄水量(mm);VWC:土壤体积含水量(%);h:土层深度(mm);∆W:表示土壤蓄水量变化量(mm);W2:降水-土壤水分响应过程后的土壤蓄水量(mm);W1:降水前的土壤蓄水量(mm)。

降水-土壤水分响应过程的土壤水分补给量计算:

$ \begin{aligned} & S_{\mathrm{p}}=\sum\nolimits_{i=1}^{10} \theta_i d_i \\ & \Delta S=S_P-S_0 \end{aligned} $ (2)

式中,SP为每次降水过程后的土壤蓄水量(mm);θi为第i个深度区间的平均容积含水量(%vol);di为第i个深度区间的厚度(mm);S0为降水前的土壤蓄水量(mm),计算方法与SP一致;∆S为每次降水-土壤水分响应过程后的土壤水分补给量(mm)。

次降水过程的水量平衡方程:

$ {P_i} = \Delta {S_i} + E{T_i} + {R_i} $ (3)

式中,Pi为每次降水过程的雨量(mm),∆Si为每次降水-土壤水分响应过程后的土壤水分补给量(mm),Ri为每次降水过程样地内产生的径流量(mm),ETi为冠层和枯落物截留(mm)。

使用Origin2021进行绘图。

2 结果 2.1 降水特征

图 2所示,研究区降水量无论在年际还是年内均存在较大变化,表现出显著的干湿交替。年内降水量呈“单峰”趋势变化,集中于6—9月,2020年8月降水量最多为391.01mm。月均降水强度与降水量的变化趋势基本一致。

图 2 观测期月尺度降水特征 Fig. 2 Monthly rainfall characteristics during the experimental period

研究期间降雨事件共计129次,参考中国气象局(标准来源为国家标准GB/T 28592-2012)将降雨划分等级,详情见表 1。(1)小雨,12 h平均降雨量0.1~4.9 mm,该类降水共10场占总场次的8%。(2)中雨,12 h平均降雨量5.0~14.9 mm,该类降水共59场占总场次的46%。(3)大雨,12 h平均降雨量15.0~29.9 mm,该类降水共34场占总场次的26%。(4)暴雨,12 h平均降雨量30.0~69.9 mm,该类降水共24场占总场次的19%。(5)大暴雨,12 h平均降雨量70.0~139.9 mm,该类降水共2场占总场次的1%。研究区大部分降水事件均属于中雨类型,尽管大雨和暴雨发生频率较低,但其在补给地下水等方面发挥着关键作用。

表 1 2019—2023年降水特征 Table 1 Rainfall characteristics during the year of 2019—2023
2.2 土壤水分变化特征

图 3所示,土壤水分变化与降水时间分布密切相关。0~100 cm的土壤水分受到降雨影响较大,随时间变化在年内呈现“双峰”型变化趋势,200~500 cm土壤水分含量相对稳定,只在降水充沛时有所增加。700~1 000 cm土壤水分含量不受降水的影响,土壤水分含量无明显变化。此外,柠条的生长也显著影响了土壤水分的变化。具体表现如下:2020年6—7月份,柠条生长的蒸散耗水量的增加,土壤水分含量显著降低,0~100 cm平均土壤含水量为6.58%和5.92%,低于凋萎系数7.82%,形成了土壤干层,8月降水量最大,0~100 cm平均土壤含水量增加为20.95%,由于降水入渗土壤具有时滞性,200cm土壤含水量在9月达到峰值,为28.43%,300 cm深土壤含水量在12月达到峰值,为18.47%。

注:萎蔫系数:宁婷等[21]在黄土丘陵区撂荒坡地研究得出萎蔫系数的均值7.82%。  Note: withering coefficient: Ning Ting et al [21]. obtained the withering coefficient of abandoned slope land of 0~400cm in loess hilly region. 图 3 观测期土壤水分含量月变化 Fig. 3 Monthly variation of soil moisture content during the experimental period

图 4所示,柠条林年际土壤水分沿剖面变化趋势一致。在0~300 cm土壤深度范围内,柠条林土壤水分趋势呈现为0~30 cm增大,30~50 cm减小,50~200 cm增大,200~300 cm减小的趋势,但由于6—7月份柠条蒸散耗水量增加,土壤水分在0~50 cm深度呈现持续减小的趋势。由于300 cm以下土层柠条根系较少,水分消耗减少,降雨充沛的季节补给深度最深为500 cm,使得在300~1 000 cm深度范围内土壤水分变化趋势为先增加,再减少,500 cm处土壤水分为16.20%~21.71%。

注:①萎蔫系数为宁婷等[21]在黄土丘陵区研究得出0~400 cm的撂荒坡地的萎蔫系数。  Note: ①withering coefficient: Ning Ting et al [21]. obtained the withering coefficient of abandoned slope land of 0~400 cm in loess hilly region. 图 4 土壤水分沿土壤剖面垂直方向的变化 Fig. 4 Changes of soil moisture along the vertical direction of soil profiles
2.3 土壤水分响应过程

为进一步探究降雨对柠条土壤水分的影响,选取2020年降水数据和土壤水分数据,分析土壤水分响应过程,划分13组降水-土壤水分响应过程并将降水划分等级,具体属性列于表 2

表 2 降水-土壤水分响应过程特征 Table 2 Characteristics of rainfall-soil moisture response process

图 5所示,不同类型的降雨导致的土壤水分响应过程也存在较大差异。降雨强度越大,土壤水分的响应越快,0~10 cm土壤对降水的响应最为敏感。暴雨(P10)和大雨(P9)的响应时间最短为3 h,其次是长历时中雨(P13)为6 h,而短历时中雨(P1)的响应过程则相对缓慢为23 h。随土层加深,响应时间也逐渐延长。土壤水分响应还受前期降水的影响,前期降水会使土壤保持在较高的含水饱和度,入渗速率加快,因此P11较P9的土壤水分响应更快。

注:P1、P13为中雨,P9、P11为大雨,P10为暴雨。  Note: P1 and P13 are moderate rain, P9 and P11 are Large rain, and P10 is heavy rain. 图 5 不同类型次降雨下土壤含水量的动态变化特征 Fig. 5 Dynamics of soil water content with time in different types of rainfall

土壤水分含量在达到最高点后便开始消退,随着深度增加土壤水分所受的环境影响减小,消退速度减慢,表现出了更强的持水能力。24 h后,10 cm处土壤水分消退最快,消退45.34%,30 cm处消退14.39%,50 cm处消退5.21%,70 cm处消退1.99%,100 cm处消退0.59%。50 cm以下的土壤在降水补给后能维持较高的含水状态,为植被生长提供了有利的水分储备。

2.4 土壤水分补给量特征

表 3所示,降水类型与土壤水分入渗深度呈显著正相关性。暴雨的降雨量、历时和雨强最大,最深入渗深度为200 cm,其次是大雨和长历时中雨,为100 cm和70 cm,短历时中雨为10 cm,小雨的入渗深度均小于10 cm。P11和P9的降水量相似,而P11的雨强和历时均较小,但水分入渗深度明显增大主要是受到前期降水的影响。降水过程的连续性也会影响水分入渗深度,P8的降水事件个数有3个,而P7和P13的降水事件个数只有2个,因此P8的入渗深度小于P7和P13。

表 3 土壤水分响应深度与降雨属性的相关性 Table 3 The Relationship correlation analysis between response depth of soil moisture and rainfall attributes

图 6所示,不同降水类型的土壤水分补给量也存在显著差异。暴雨(P10)的土壤水分补给量最大,占降水量的87.5%,其中50 cm以下补给量为249.4 mm,占暴雨补给量的82.1%,其次是大雨,为36.2%,长历时中雨为29.7%,短历时中雨为12%,小雨未能使土壤水分得到补给。由此可见,暴雨作为最有效的补给来源,在该区域土壤水分补给中发挥着不可替代的作用。

注:P2、P5为小雨,P1、P3、P4、P7、P8、P12、P13为中雨,P6、P9、P11为大雨,P10为暴雨。  Note: P2、P5 are light rain, P1、P3、P4、P7、P8、P12、P13 are moderate rain, P6、P9、P11 are large rain, and P10 is heavy rain. 图 6 不同降水模式土壤水分入渗深度及各深度土层的水分补给量 Fig. 6 Infiltration depth of soil water in different rainfall patterns and water supply of soil layers at different depths

根据不同土层的萎蔫系数和最大入渗深度值,计算得出该地土壤水资源利用限度为154.6 mm。6—7月,土壤水资源总量低于土壤水资源利用限度,土壤出现了季节性干层,柠条根系的吸水困难,土壤水分抑制柠条的生长。但在研究区降雨量较大的时期,降水的补给使土壤水资源的总量上升,满足了柠条生长所需的水分。因此,虽然在夏季出现了季节性干层,但年降雨量总体上可以满足柠条生长所需。

表 4列出了不同降水类型水量平衡分配情况。暴雨产流量最大,有利于当地河流健康,暴雨和长历时中雨的补给最多,有利于当地柠条生长,而小雨、短历时中雨和大雨则大部分以蒸散发的形式损失。因此,及时高效地利用暴雨资源补给深层土壤,是缓解区域干旱、恢复生态系统的关键举措。

表 4 次降水过程的水量平衡特征 Table 4 Characteristics of water balance in a rainfall process
3 讨论 3.1 柠条人工林地土壤水分动态变化

降水格局与土壤水分的分布状况有着密切关系,降水峰值往往会引起土壤水分峰值的出现,同时植被生长的节律和气温的季节变化进一步促进了土壤水分的动态变化[21-22]。目前有学者[23-25]的研究结果发现各月的土壤水分与降水量显著正相关,0~100 cm土壤水分存在显著季节性变化,年内变化基本经历由少至多2次交替,在每年4月和9月左右是土壤水分含量的高值中心。本研究的研究结果(图 3)与上述研究结果一致,柠条0~100 cm土壤水分在年内变化表现出明显的“双峰”趋势,主要原因为:(1)春季气温回升,冰雪融水对浅层土壤进行了补给;(2)秋季降水较为充沛,为土壤补给了较多水分,且秋季气温降低,柠条蒸腾速率下降,水分消耗减小,因此秋季是全年土壤水分恢复最充足的时期,含水量可达到全年最高点。200~500 cm土壤水分含量相对稳定,只在降水充沛时有所增加,但由于降水入渗土壤具有时滞性[26-27],深层土壤水分补给较慢,但水分含量稳定。700~1 000 cm土壤水分含量基本不受降水的影响,随深度的增加柠条土壤水分含量无明显变化。

土壤水分垂直空间分布受土壤蒸发、植被蒸腾、根系分布及土壤类型的影响会表现出一定的差异性,有研究发现随着深度的增加,土壤体积含水量表现出先增加—后减小—再增加的趋势[28-31]。本研究结果也显示类似的结论(图 4),年内土壤水分在0~300 cm深度范围内均呈现先增大后减小再增大再减小的趋势,其主要是受到大气降水和蒸散发的影响以及植被根系调节[32]。由于300 cm以下土层柠条根系较少,水分消耗减少,降雨充沛的季节补给深度最深为500 cm,使得在300~1 000 cm深度范围内土壤水分变化趋势为先增加,后持续减小的趋势。

在柠条生长旺盛期,由于降水的高度季节性集中和土壤水分滞后响应的双重影响,50~100 cm出现季节性干层(图 3图 4)。但在雨季,降水补给会使这些缺水层次得到恢复,土壤水分含量达到柠条萎蔫系数之上,满足柠条用水需求,因此年降水量总体满足柠条生长所需。

3.2 土壤水分含量对降水响应的差异

土壤水分对降水的响应受到多种因素的影响,但降水的雨量和强度对土壤水分的影响最为显著。降水量和降水强度越大,水分入渗的深度就越深,补给量也越多[33-35]。本研究结果(图 6)显示,暴雨入渗深度最深可达200 cm,而小雨的入渗深度仅限于0~10 cm。暴雨的土壤水分补给量最大,占降水量的87.5%,其次是大雨,为36.2%,长历时中雨为29.7%,短历时中雨为12%,小雨未能使土壤水分得到补给。除了当次降水的特征外,前期降水状况也会显著影响水分入渗深度,当表层初始含水量较高时,降水入渗会加快,持续时间短,补给作用大。如图 5图 6所示,P11与P9相比,二者降水量、降水强度和降水历时相似,但P11入渗深度显著增大,入渗速度更快。这是由于土壤处于一个较为湿润的状态,不仅有利于湿润锋向下运移,还会通过增强土壤气体效应而加速这一过程[36]

不同土层对降水的响应存在明显差异,降水响应的程度也将随土层深度的增加而逐渐减弱[37-38],本研究结果显示(图 4)0~100 cm深度的土壤水分波动幅度最大,其中以10 cm土壤水分最为敏锐地反映出降水量的变化,0~100 cm深度的土壤水分受到降水量和强度的影响最为显著。200~300 cm土壤水分的变化则介于两者之间,受植物根系的影响较大。由于远离地表,300 cm以下的土壤水分的波动十分有限,仅在极端降水事件下有所响应。随着深度的加大,土壤水分受蒸发损耗的影响越来越小,因此,对于深层土壤而言,一旦接受了降水补给,其含水量就能维持较长时间,为干旱时期提供一定的缓冲能力。从水量平衡的角度分析,降水量在入渗和蒸发之间的分配将决定着干旱黄土的恢复速度,研究结果表明,无论什么样的降水类型其主要份额仍以补给土壤水分为主。

4 结论

研究区大部分降水事件均属于中雨类型,暴雨虽然发生频率较低,但在补给土壤水分等方面发挥着不可替代的作用。黄土丘陵区柠条林地土壤水分动态呈现明显的季节性变化特征,主要源于降水时空分布格局和植被蒸散发的影响。具体表现为:土壤水分在年内变化呈现“双峰”型变化,与当地的雨季和干季交替出现密切相关。从垂直分布上看,0~100 cm土壤水分波动最为剧烈,200~300 cm次之,500~1 000 cm则相对稳定。夏季柠条生长旺盛期,50~100 cm土层普遍会出现不同程度的季节性干旱,会对植被的正常生长造成不利影响,但年降水量总体上可以满足柠条生长所需。降水类型是影响土壤水分入渗深度和补给量的关键因素。暴雨事件的入渗深度最深,可达200 cm,而小雨仅局限于0~10 cm;中雨则介于两者之间,随降水历时的增加,入渗深度也逐步加大。暴雨的土壤水分补给量远高于小雨和中雨,占降水量的87.5%,而小雨基本无法实现有效补给。降水的主要份额以补给土壤水分为主,暴雨、大雨和历时长中雨能较好对土壤水分进行补给,小雨和短历时中雨补给效率较低,更多以蒸散发形式损失。

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