Temporal Stability Of Soil Water Content Research Articles

Response of soil water content temporal stability to stand age of Haloxylon ammodendron plantation in Alxa Desert, China.

Afforestation as an effective measure for wind and sand control has achieved remarkable results in northern China, and has also greatly changed the land use and vegetation characteristics of the region. It is important to study the spatial and temporal dynamics of soil water content (SWC) in different afforestation years and its temporal stability to understand the dynamic characteristics of SWC during afforestation. In order to reveal the spatiotemporal dynamic characteristics of SWC in desert area Haloxylon ammodendron (HA)plantations, in this study, five restorative-aged HA plantations in desert areas were selected and their SWC was measured in stratified layers for the 0-400 cm soil profile; we also analyzed the spatiotemporal dynamics and temporal stability of the SWC. The results showed that the SWC of HA plantations decreased with the increase in planting age in the measurement period, and the SWC of deep layers increased by more than that of shallow layers with planting age. Spearman's rank correlation coefficients for SWC of 0-400 cm in both 5- and 11-year-old HA plantations reached above 0.8 and were highly significantly correlated; the temporal stability of SWC tends to increase as the depth of the soil layer deepens. In contrast, the temporal stability of SWC in deeper layers (200-400 cm) of 22-, 34- and 46-year-old stands showed a decreasing trend with depth. Based on the relative difference analysis, representative sampling points can be selected to monitor the regional average SWC, but for older HA plantations, the uncertainty factor of stand age should be considered in the regional moisture simulation. This study verified that it is feasible to simulate large-scale SWC in fewer observations for HA plantations younger than 11 years old, while large errors exist for older stands, especially for deeper soils. This will help soil moisture management in HA plantations in arid desert areas.

Efficient prediction of profile mean soil water content for hillslope-scale Caragana korshinskii plantation using temporal stability analysis

Spatio-temporal variability in soil water content (SWC) increases the difficulty in soil water resource management and vegetation construction in arid and semi-arid regions. Our objectives were to investigate the spatial variability and temporal stability of SWC and identify representative locations for reliably predicting the profile mean SWC (MSWC) of a 33-year-old Caragana korshinskii plantation on typical sunny and shady hillslopes on the Loess Plateau, China. The profile SWC (0–180 cm) in three sampling belts and five hillslope positions (upper position, UP; upper-middle position, UMP; middle position, MP; lower-middle position, LMP; lower position, LP) were measured 27 times for both the hillslopes in 2018 and 2019. The mean relative difference (MRD), the standard deviation of the MRD (SDRD), and an index of temporal stability (ITS) were used to identify representative locations of the MSWC. The results indicated that the SWC presented different horizontal but similar vertical change trends in the five hillslope positions. The profile MSWC on the sunny hillslope (8.89%) was lower than that on the shady hillslope (9.44%). The mean Spearman correlation coefficients of the temporal patterns ranged from 0.6 to 0.8 for the five soil layers on both hillslopes. The characteristics of temporal stability differed in both temporal persistence and the representative locations for the five soil layers on the sunny and shady hillslopes. The mean ITS values generally decreased with increasing soil depth. Ultimately, the representative locations for estimating the profile MSWC were determined to be the MP-2 location on the sunny hillslope and the MP-3 location on the shady hillslope based on the MRD, SDRD, and ITS values. The representative locations have the lowest ITS values and represent the MSWC accurately, with root mean square error (RMSE) and mean error (ME) values of less than 1%. In addition, the soil organic carbon, clay, silt, and sand contents greatly affect the spatial and temporal distributions of the soil water at the hillslope scale. Our study provides a rapid and accurate method for determining profile MSWC in C. korshinskii plantations on typical sunny and shady hillslopes and could also support valuable guidance for the rational management of hillslope-scale SWC resources for similar dry regions.

Use of montmorillonite-enriched siltstone for improving water condition and plant growth in sandy soil

Revegetation in arid and semi-arid regions is restricted by soil water shortage. Improvement of sandy soil properties, especially for soil water related properties, is therefore vital for successful revegetation. In this study, montmorillonite-enriched siltstone (MS), a type of fine particle soil material, was used to improve sandy soil in a field experiment with four volume proportions: 0% (M0%), 25% (M25%), 75% (M75%), and 100% (M100%) of MS, which was then planted with alfalfa (Medicago sativa) for 3 years. The objective was to investigate the improvement effect of MS on the physical properties of sandy soil and its support for plant growth. The results showed that the addition of MS significantly increased the soil physical quality (SPQ) indicators, such as available water capacity (AWC), which increased from 7.39% (M0%) to 10.18% (M100%), while hydraulic conductivity (Ks) decreased by 34.1% (M25%) and 57.5% (M75%), and bulk soil air capacity (ACB) decreased by 12.6% (M25%) and 39.4% (M75%). These results were mainly related to the high content of fine particles in MS, such as montmorillonite, a 2:1 clay mineral. The average profile soil water content (SWC) increased from 9.20% to 14.08% (M25%) and 24.62% (M75%). The temporal stability of SWC was also significantly increased by MS addition, while drainage was significantly decreased. Both above-ground biomass (ABG) and water use efficiency (WUE) increased by approximately two times in M25% and M75% in 2015 and five times in M25% and M75% in 2016. This result verified the positive effect of MS addition on sandy soil properties. However, more than 75% MS addition (such as M100%) in sandy soil decreased ACB to less than 15% and decreased ABG by 75.3% compared with that of M75%. This study gives a new strategy for improving sandy soil using local finer materials with a high content of montmorillonite. Considering the improvement effects and engineering cost, a 25% volume proportion of MS was recommended.

The role of sampling strategy on apparent temporal stability of soil moisture under subtropical hydroclimatic conditions

Abstract Subtropical regions have clay-rich, weathered soils, and long dry periods followed by intense rainfall that produces large fluctuations in soil water content (SWC) and hydrological behavior. This complicates predictions of spatio-temporal dynamics, as datasets are typically collected at too coarse resolution and observations often represent a duration that is too short to capture temporal stability. The aim of the present study was to gain further insights into the role of temporal sampling scale on the observed temporal stability features of SWC order to aid the design of optimal SWC sampling strategies. This focused on both sampling frequency and total monitoring duration, as previous analyses have not considered both of these sampling aspects simultaneously. We collected relatively high resolution data of SWC (fortnightly over 3.5 years) for various soil depths and under contrasting crops (peanuts and citrus) at the red soil region of southeast China. The dataset was split into a three-year training period and a six-month evaluation period. Altogether 13 sampling frequencies (intervals ranging from 15 to 240 days) and eight monitoring duration periods (between three and 36 months) were derived from the training period to identify temporal stability features and the most time stable location (MTSL). The prediction accuracies of these MTSLs were tested using the independent evaluation data. Results showed that vegetation type did affect the spatio-temporal patterns of SWC, whereby the citrus site exhibited a stronger temporal variation and weaker temporal stability than the peanut site. However, the effects of both sampling frequency and observation duration were more pronounced, irrespective of the role of vegetation type or soil depth. With increasing sampling interval or decreasing monitoring duration, temporal stability of SWC was generally overestimated; by less than 10% when sampling interval increased from every 15 to 240 days and by up to 40% with duration decreasing from 36 to 3 months. Our results suggest that sampling strategies and trade-offs between sampling interval and duration should focus on capturing the main variability in hydro-climatological conditions. For subtropical conditions, we found that sampling once every 45 days over 24 months to be the minimum sampling strategy to ensure errors in SWC temporal stability of less than 10%.

Spatiotemporal characteristics and temporal stability of soil water in an alpine meadow on the northern Tibetan Plateau

The spatiotemporal characteristics of soil water content (SWC) have a significant influence on vegetation degradation and growth in alpine meadow ecosystems. The spatiotemporal variability and temporal stability (TS) of SWC, however, have rarely been studied on the northern Tibetan Plateau owing to the rugged and hostile sampling environment. The objective of this study was to analyze the spatiotemporal variability and the TS of SWC in various layers of the soil, to a depth of 50 cm in a 33.5 hm2 plot, with the data obtained from 113 measuring locations collected on 22 sampling occasions during the growing seasons of 2015 and 2016. The SWC was moderately variable both in time (two consecutive growing seasons) and horizontal space (plot). The variabilities, however, did not vary consistently with increasing depth for the various dominant influencing factors. The SWC in the undeveloped, shallow, and stony alpine meadow soil was temporally stable; TS did not depend on depth due to disturbances by grass roots...

Long-term temporal variations of soil water content under different vegetation types in the Loess Plateau, China

The temporal characteristics of soil moisture are critical for the stability and sustainability of rebuilt ecosystems. In China's Loess Plateau (CLP), large areas of unsuitable croplands were restored to forest or grassland in order to control soil and water loss. Thus, knowledge of the temporal pattern and its controlling factors of soil moisture in revegetated areas over long-term is indispensable. In this study, four plots with different land uses (shrubland, grassland, fallow land and cropland) were set up on a loess slope in northern CLP. The profile soil moisture in the 0–4m soil layer was measured from 2004 to 2015. Semivariogram models were established to analyze the profile temporal characteristics of soil water content (SWC) over 12years of revegetation. The results suggested that: temporal stability of SWC under cropland was better than that under the three types of revegetated lands. Among the three types of revegetated lands, fallow land had stronger stability and higher value of SWC and was thus recommended as the best revegetation type. Generally, variation of SWC decreased with the increase of soil depth under shrubland, fallow land and cropland. However, under grassland, an inverse trend of SWS variation was observed and huge water absorption by deep grass roots was responsible for this trend. The temporal variation in SWC was under the joint influence of land use, soil depth and slope position, and land use was regarded as the dominant one. The study provided wider insight into scientific and reasonable vegetation restoration strategies in CLP region.

Application of temporal stability analysis in depth-scaling estimated soil water content by cosmic-ray neutron probe on the northern Tibetan Plateau

Soil moisture is a key limiting factor in grass growth and restoration in alpine meadow ecosystems on the northern Tibetan Plateau. In the deeper layers, soil moisture influences the processes of freeze-thaw, erosion and water cycle. Cosmic-ray neutron probe (CRNP) is a new method for continuously monitoring mean soil water content (SWC) at hectometer scale, which has been applied in an alpine meadow at a high accuracy. However, with CRNP measuring depth of only 30cm, depth-scaling is needed for sufficient insight into deep-layer soil moisture. This study evaluated the accuracy of CRNP measurement of SWC in the 2015 and 2016 growing seasons and the performance of temporal stability (TS) analysis in depth-scaling CRNP-estimated SWC. During the study period, 11 field samplings were done for calibration of CRNP-estimated SWC. Using 22 occasions of neutron probe measurements for each of 113 investigated locations, the TS of SWC was analyzed and its performance in depth-scaling CRNP-estimated SWC at five soil depths (10, 20, 30, 40 and 50cm) was evaluated. The results showed that the mean SWCs to the depth of 50cm were 12.9 and 17.0%, respectively in 2015 and 2016 growing seasons and were temporally influenced by precipitation and spatially by soil depth. The accuracy of the CRNP-measured SWC was high, with root mean square error and Nash-Sutcliffe efficiency coefficient (NSE) of 2.1% and 0.832, respectively. Representative locations for TS existed in all the soil layers, which increased with increasing soil depth. For the various soil layers, TS-estimated SWC was close to field-measured value. Only a relatively small error and high NSE were noted, suggesting that TS was reliable in application in CRNP depth-scaling. The study provided further scientific basis for the application of CRNP and an effective way of depth-scaling CRNP-estimated mean SWC in alpine meadow ecosystems.

Profile distribution of soil–water content and its temporal stability along a 1340-m long transect on the Loess Plateau, China

Information on the profile characteristics of soil–water content (SWC) and its temporal stability is essential for an accurate understanding of hydrological processes. This study investigated changes of spatial variation and temporal stability of SWC in soil profiles and estimated mean SWC based on direct and indirect methods. SWCs were measured at 20-cm intervals in the soil profiles to a depth of 3m using neutron probes at 135 locations along a 1340-m long transect on 18 sampling dates from 2012 to 2013 on the Loess Plateau in China. The coefficient of variation over space (CVS) of SWC first decreased and then increased vertically. The coefficient of variation over time (CVT) of SWC decreased along the soil profiles. The spatial patterns of SWC strongly persisted vertically and temporally. Mean Spearman's rank correlation coefficients decreased from a depth of 10 to a depth of 20cm, fluctuated from 20 to 180cm, and then increased below 200cm. Temporal stability increased along the soil profiles based on the standard deviation of mean relative difference (SDRD) and the mean absolute bias error (MABE). The number of locations with an SDRD and/or MABE <5% increased along the soil profile, and the number of locations with a mean relative difference within ±5% and/or representative locations were variably dependent on depth. Both direct and indirect methods could accurately estimate the mean SWC for each depth based on the mean absolute relative errors and root mean square errors. The driest and wettest locations tended to remain representative for more depths than did locations with mean-moistures. The driest locations were more likely to be the most temporally stable. These findings should improve our understanding of soil–water dynamics in soil profiles.

Spatio-temporal variability and temporal stability of water contents distributed within soil profiles at a hillslope scale

Information about soil water content (SWC) within soil profiles in terms of its spatio-temporal variability and temporal stability is crucial when selecting appropriate soil water management practices. However, detailed profile distribution features for related indices are not clear on the Chinese Loess Plateau. This study aimed to investigate the depth dependency of spatio-temporal variability and temporal stability of SWC at the hillslope scale using an intensive sampling strategy for both horizontal and vertical directions as well as over time. The SWCs at 20 depths within 0–300cm soil profiles were measured on 20 occasions between July 2008 and October 2010 at 91 locations on a hillslope on the Loess Plateau, China. Results showed that although the profile distributions of investigated statistical parameters differed greatly, they were all depth dependent. Based on both the spatio-temporal variability and the temporal stability characteristics using eight indices, the studied soil profile (0–300cm) could be divided into three soil layers, i.e., 0–60, 60–160 and 160–300cm, with a characteristic feature of “irregularly changing”, “regularly changing”, and “relatively constant”, respectively. The deepest rainwater replenishment depth was approximately 160cm. Therefore, choosing 200cm as the sampling depth would be sufficient in similar areas. Such findings are useful for designing an optimal strategy for sampling and management of profile soil moisture.

Estabilidade temporal de medidas do teor e do potencial mátrico da água no solo em uma transeção

O elevado nível de variabilidade espacial de observações experimentais de campo da condutividade hidráulica do solo dificulta o planejamento de amostragem e a utilização e extrapolação de valores médios, em estudos de campo de dinâmica da água no solo. Nesse sentido, o objetivo deste trabalho foi verificar a estabilidade temporal de medidas do teor e do potencial mátrico da água no solo e sua aplicação como ferramenta para racionalizar a estratégia de amostragem para avaliação da condutividade hidráulica do solo em condições de campo. O experimento de drenagem foi desenvolvido num Latossolo Vermelho-Amarelo em Piracicaba (SP), numa parcela experimental com 50 pontos de observação distanciados de 1 m entre si, nos quais foram medidos o teor de água (com TDR) e o potencial mátrico (com tensiômetros) durante três semanas de redistribuição da água no solo. Os resultados mostraram que o método da estabilidade temporal permite identificar, com exatidão, os locais mais adequados para as amostragens, possibilitando a redução do número de amostras necessárias e do custo de execução do esforço amostral para o planejamento de sistemas de uso e manejo de água na agricultura.