Vertical Distribution Of Soil Moisture Research Articles

Dynamic root growth in response to depth-varying soil moisture availability: a rhizobox study

Abstract. Plant roots are highly adaptable, but their adaptability is not included in crop and land surface models. They rely on a simplified representation of root growth, which is independent of soil moisture availability. Data of subsurface processes and interactions, needed for model setup and validation, are scarce. Here we investigated soil-moisture-driven root growth. To this end, we installed subsurface drip lines and small soil moisture sensors (0.2 L measurement volume) inside rhizoboxes (length × width × height of 45 × 7.5 × 45 cm). The development of the vertical soil moisture and root growth profiles is tracked with a high spatial and temporal resolution. The results confirm that root growth is predominantly driven by vertical soil moisture distribution, while influencing soil moisture at the same time. Besides support for the functional relationship between the soil moisture and the root density growth rate, the experiments also suggest that the extension of the maximum rooting depth will stop if the soil moisture at the root tip drops below a threshold value. We show that even a parsimonious one-dimensional water balance model, driven by the water input flux (irrigation), can be convincingly improved by implementing root growth driven by soil moisture availability.

Soil moisture and salinity dynamics of drip irrigation in saline-alkali soil of Yellow River basin

Soil secondary salinization in the Yellow River Diversion Irrigation Area of Northwest China seriously threatens local agricultural production. Drip irrigation technology is one of the largest contributors to low-yielding saline-alkali land; however, research on the high spatio-temporal scale variability of soil moisture and salinity in drip irrigation is still lacking. Herein, four treatments, CK (flood irrigation, 900 mm), W1 (small volume drip irrigation, 360 mm), W2 (medium volume drip irrigation, 450 mm), and W3 (large volume drip irrigation, 540 mm), were set up to investigate the characteristics and laws of soil moisture and salinity under different irrigation methods. The results showed that the soil moisture of drip irrigation was 5.02%–17.88% (W1), 7.36%–21.06% (W2), and 13.79%–27.88% (W3) higher than that of flood irrigation, resulting in a vertical distribution of soil moisture being low at the top and high at the bottom. Under drip irrigation, the soil salinity formed a desalination zone centered on the drip emitter and this zone gradually expanded to deeper soil with continuous drip irrigation, gradually transforming the soil from surface aggregation type to the bottom accumulation type. The desalination rates of W1, W2, and W3 were 18.46%, 20.84%, and 22.94%, respectively, whereas the salt leaching rate of CK was slower and the salt distribution was not uniform; therefore, the desalination rate was only 5.32%. By precisely controlling the irrigation water volume and flow, drip irrigation significantly reduced surface evaporation and subsurface leakage of water and improved water use efficiency, thus increasing grain yield. Compared with flood irrigation, the yield increase rates of W1, W2, and W3 were 6.6%, 16.18%, and 18.32%, respectively. Therefore, drip irrigation with an appropriate irrigation volume in the saline land in northern Ningxia can improve water saving, salt suppression, and maize yield.

Impacts of moss-dominated biocrusts on rainwater infiltration, vertical water flow, and surface soil evaporation in drylands

• Biocrusts have higher water-holding ability and capacity as compared with bare soils. • Biocrusts decrease homogeneous matrix flow and infiltration depth during penetration. • Biocrusts increase soil moisture both at surface and subsurface soils after infiltration. • Biocrusts raise evaporation rate by 27%−39% and cumulative evaporation amount by 14% • Depth-restricted infiltration and wetter surface soil enhance biocrusts evaporation. Globally widespread biocrusts form a “living skin” on most dryland soils, helping govern fundamental ecosystem functions, particularly hydrological processes. However, the underlying mechanisms of biocrust effects on soil water infiltration and evaporation are still controversial. Biocrusts possibly regulate infiltration and evaporation due to their unique physicochemical properties and influence on surface soil moisture ( θ ), and these assumptions were verified in this study conducted on the Loess Plateau of China. The differences in vertical soil moisture distribution and flow patterns of infiltration water between moss-dominated biocrusts and bare soil were investigated through dye tracing experiments. The evaporation characteristics of biocrusts and bare soil were also obtained by weighing micro-lysimeters in both simulated and in-situ experiments. Results showed that biocrusts retarded deep soil water infiltration below ∼ 15 cm and significantly ( F = 48.15, P < 0.001) decreased the stained area ratio (percentage of stained area to total area at the same soil depth) by 16% in contrast to bare soil. Because the infiltrated water was mostly retained in shallow soil (0–15 cm) in biocrust-covered soil, their ratios of low interaction flow (stained path width < 20 mm) and high interaction flow (20 mm < stained path width < 200 mm) were significantly higher ( F ≥ 7.82, P ≤ 0.007) than those of bare soil. After the dye tracing experiments, the biocrusts modestly increased θ by 17% throughout the soil profile (0–50 cm), but they remarkably increased θ by 33%−78% in the top 20 cm of soil. Additionally, the biocrusts increased soil daily evaporation rate ( E ) by 27%−39% (with only the exception of the beginning) and the cumulative evaporation amount by 14% in contrast to bare soil over an 8-day field measurement. More importantly, the biocrust-affected soil properties, such as the contents of fine particles and organic matter, as well as saturated water content and field capacity, were closely correlated with E (| ρ| ≥ 0.52, P ≤ 0.08). Consequently, it was concluded that biocrusts restrict the depth of rainwater infiltration and increase surface θ , which may greatly enhance surface soil evaporation in the Chinese Loess Plateau.

Quantifying the impacts of agricultural alteration and climate change on the water cycle dynamics in a headwater catchment of Lake Urmia Basin

The rapid shrinkage of Lake Urmia over the past two decades has raised serious environmental concerns. Several restoration plans have been proposed to reduce agricultural water consumption to supply a remarkable fraction of the lake’s environmental flow requirement. However, an accurate and realistic evaluation of the effectiveness of these plans in reducing agricultural water consumption across the basin is still poorly understood. This study assessed the water-saving potential of agricultural alteration in a snow-dominated catchment in the Lake Urmia Basin. In particular, we explored the impact of crop pattern alteration via reducing the cultivation area and employing different irrigation systems. We further studied the degree of extra stress on the agricultural water demand under the future trends of air temperature and precipitation due to climate change. To this end, we developed a three-dimensional physically-based hydrological model that simulated the main processes involved in the water and energy cycles. In this setting, a surface-groundwater model was coupled to a land-surface model that integratively computed the spatiotemporal variability of evapotranspiration, snowmelt, groundwater, and soil moisture in the root zone, among other processes. Pumping from wells and different irrigation systems were also included in the model. The results showed that a reduction in the cultivation area does not necessarily lead to a significant saving in the total water loss due to evapotranspiration, which strongly depends on the trade-off between the decrease in transpiration and the increase in soil evaporation. We further found that saving in agricultural water consumption is partly controlled by topography and the vertical distribution of soil moisture. Also, the drip irrigation system showed lower evapotranspiration rates than the spray irrigation system. Finally, the study of the future impacts of climate change revealed that the agriculture sector would be under more intensive stress, which poses a severe threat to the long-term restoration of Lake Urmia if effective agricultural management practices are not taken into consideration. The findings of this study highlight important issues for the Lake Urmia restoration plans on agricultural water management. Given the fragility of the lake ecosystem, this study suggests a reliable modeling framework for water and agricultural managers to identify the most appropriate regions in the basin that would yield the highest saving in agricultural water consumption by improving the current crop patterns and irrigation systems.

Investigation of the Possibility of Extending Soil Moisture Sensor Data

Smart irrigation requires soil moisture (SM) data in order to determine the water needs of each point within the agricultural field. The aim of the study is to develop methodologies for testing different approaches and to present the provisional results. A complex methodology was applied to process and analyze the sensor data. First spatial variability was assessed using regression and geostatistical interpolation. A vertical distribution estimation was performed by extending the SM values of each sensor for all their depth levels. The latest results of this method show that the statistical correlations between the different depths can be used to characterize the soil types and their corresponding vertical SM distribution.

High‐Resolution Soil Moisture Evolution in Hyper‐Arid Regions: A Comparison of InSAR, SAR, Microwave, Optical, and Data Assimilation Systems in the Southern Arabian Peninsula

AbstractWe present an analysis of a 3‐year time series of Synthetic Aperture Radar (SAR) imagery covering the southern Arabian Peninsula, spanning two devastating typhoons that impacted the region in 2018. The limited vegetation and precipitation in the area allows for clear separation of soil moisture changes from other factors that typically also affect microwave imagery. We generate soil moisture proxies from Interferometric SAR (InSAR) coherence and compare them with a range of other soil moisture products. We estimate the decay timescale over which the surface returns to its pre‐storm state for each event and data type, and find that the radar coherence‐based timescales tend to be longer than those for the other products. We also find that the coherence‐based timescales of the two events differ from each other in a manner that is consistent with seasonal temperature fluctuations and the total rainfall that occurred each time. The higher spatial resolution of the InSAR coherence results and differing sensitivity to the vertical distribution of soil moisture complements the other more temporally dense products, particularly in regions of high heterogeneity along the edge of the storm and within regions where the surface properties change dramatically over small spatial scales. As the frequency and strength of tropical cyclones increase, and as already scarce water resources are utilized further in arid regions, InSAR coherence data constitutes another tool that researchers can use to expand our understanding of these challenges.

The Influence of Desertification Pattern on Vertical Distribution of Soil Moisture in Acacia Senegalese

The Influence of Desertification Pattern on Vertical Distribution of Soil Moisture in Acacia Senegalese

Promise and pitfalls of modeling grassland soil moisture in a free-air CO2 enrichment experiment (BioCON) using the SHAW model

Free-air carbon dioxide (CO2) enrichment (FACE) experiments provide an opportunity to test models of heat and water flow under novel, controlled situations and eventually allow use of these models for hypothesis evaluation. This study assesses whether the United States Department of Agriculture SHAW (Simultaneous Heat and Water) numerical model of vertical one-dimensional soil water flow across the soil-plant-atmosphere continuum is able to adequately represent and explain the effects of increasing atmospheric CO2 on soil moisture dynamics in temperate grasslands. Observations in a FACE experiment, the BioCON (Biodiversity, CO2, and Nitrogen) experiment, in Minnesota, USA, were compared with results of vertical soil moisture distribution. Three scenarios represented by different plots were assessed: bare, vegetated with ambient CO2, and similarly vegetated with high CO2. From the simulations, the bare plot soil was generally the wettest, followed by a drier high-CO2 vegetated plot, and the ambient CO2 plot was the driest. The SHAW simulations adequately reproduced the expected behavior and showed that vegetation and atmospheric CO2 concentration significantly affected soil moisture dynamics. The differences in modeled soil moisture amongst the plots were largely due to transpiration, which was low with high CO2. However, the modeled soil moisture only modestly reproduced the observations. Thus, while SHAW is able to replicate and help broadly explain soil moisture dynamics in a FACE experiment, its application for point- and time-specific simulations of soil moisture needs further scrutiny. The typical design of a FACE experiment makes the experimental observations challenging to model with a one-dimensional distributed model. In addition, FACE instrumentation and monitoring will need improvement in order to be a useful platform for robust model testing. Only after this can we recommend that models such as SHAW are adequate for process interpretation of datasets from FACE experiments or for hypothesis testing.

Study on Soil Moisture Temporal-Spatial Variation and Stability in Loess Plateau of China

Soil moisture temporal-spatial distribution and stability are important factors influencing soil pollutant transport, material migration, regional climate change, hydro-ecological environment as well as agricultural production patterns. According to soil moisture distribution features on the loess plateau, the paper selected three different kinds of surface slopes on the typical experiment sites to carry out soil moisture measurement. The variation coefficient and rank correlation method are proposed to calculate the longitudinal and vertical soil moisture distribution characteristics. The temporal-spatial dynamic features of soil moisture are studied. The present findings show that the soil moisture in the Loess Plateau varies greatly at different soil depths during the period from April to June. The maximum soil moisture appears at the depth of 40-60 cm. The variation coefficient of soil moisture increases with the soil depth. It was discovered that there exists temporal stability at the depth of 20-80cm. The correlation coefficient of soil moisture in different slopes is significant during the period of April-June. The results would play an important support in theoretical research and ecological environment planning, optimization of agricultural planting pattern and comprehensive management of Loess Plateau.

Vertical distribution and seasonal variation of soil moisture after drip-irrigation affects greenhouse gas emissions and maize production during the growth season

Providing enough food for the increasing global population is difficult due to water shortages, which can be partially resolved by regulating soil moisture. Soil moisture influences soluble nutrient uptake and microbial activity, which determine crop growth, but also affects greenhouse gas (GHG) emissions. Farming is increasingly contributing to GHG emission, but little is known about the effects of the vertical soil moisture distribution on GHG or maize (Zea mays L.) yield over the growth season. In this study, there were five irrigation treatments: no irrigation (NI), and irrigation of the top (0–30 cm) (TI), middle (30–60 cm) (MI), bottom (60–90 cm) (BI), and all (0–90 cm) (AI) soil layers. The results showed that TI, MI, BI, and AI increased CO2 (25–60%), CH4 (80–270%), and N2O (17–96%) emissions, and the global warming potential (25–63%), while also increasing grain yield (13–119%) and reducing GHG intensity by 12–27%. While higher soil moisture in the shallow soil layer increased grain yield and GHG emissions, GHG intensity was lowest. Subsurface irrigation or control of the “drip irrigation interval” improve grain yield and resource use efficiency with lower environmental costs contributing agricultural sustainable development.

Effect of Rainfall Events on SMAP Radiometer-Based Soil Moisture Accuracy Using Core Validation Sites

AbstractSoil moisture retrieval is particularly challenging during and immediately after precipitation events because of the transient movement of water in the shallow subsurface. Conventional L-band microwave radiometer–based soil moisture products use algorithms that assume a static state and a constant vertical soil moisture distribution. This study assessed the retrieval performance of a SMAP radiometer-based soil moisture product during and immediately after rain events. The removal of the rain event samples systematically improved the unbiased root-mean-square error (ubRMSE) from 0.037 (all measurements) to 0.028 m3 m−3 (transitory measurements screened out), while the magnitude of the bias became larger (from −0.005 to −0.014 m3 m−3); RMSE improved from 0.047 to 0.042 m3 m−3, and the Pearson correlation saw a minor positive change from 0.813 to 0.824. The results indicate that removing samples during the transitional period causes the comparison to improve, but also suggests that the true bias may be larger than the one estimated using all the samples. Furthermore, the results revealed that the effect was stronger for areas with high clay content. An assessment of the performance of the product during the rain events (overpass within 3 h from the start of the rain) showed that the ubRMSE degraded from the benchmarked 0.036 m3 m−3 (during no rain events at all) to 0.043 m3 m−3 (during rain). The results also showed that the bias became wetter, which is expected because SMAP sensed the water on the surface before propagating to the in situ sensors. SMAP maintains its soil moisture sensitivity even during rain events and screening of rain events may not be necessary to ensure sufficient soil moisture retrieval quality.

Earthworm activity and availability for meadow birds is restricted in intensively managed grasslands

Abstract Earthworms are an important prey for the endangered meadow birds of northwest Europe. Although intensive grassland management with high manure inputs generally promotes earthworm abundance, it may reduce the effective food availability for meadow birds through desiccation of the topsoil, which causes earthworms to remain deeper in the soil. We studied the response of Red Worm Lumbricus rubellus, a detritivore, and Grey Worm Aporrectodea caliginosa, a geophage, to soil moisture profiles in the field and under experimental conditions. Surfacing earthworms were counted weekly in eight intensively managed grasslands (treated with high inputs of slurry by slit injection) with variable groundwater tables in the Netherlands. At each count, soil penetration resistance, soil moisture tension and groundwater level were measured, while air temperature and humidity were obtained from a nearby weather station. The response to variation in the vertical distribution of soil moisture was also experimentally studied in the two earthworm species. In the field, earthworms’ surfacing activity at night was negatively associated with soil moisture tension and positively by relative air humidity. Surprisingly, there was no effect of groundwater level; an important management variable in meadow bird conservation. Under experimental conditions, both L. rubellus and A. caliginosa moved to deeper soil layers (&gt;20 cm) in drier soil moisture treatments, avoiding the upper layer when moisture levels dropped below 30%. Synthesis and applications. We propose that in intensively managed grasslands with slurry application, topsoil desiccation reduces earthworm availability for meadow birds. This can be counteracted by keeping soil moisture tensions of the top soil above −15 kPa. We suggest that the late raising of groundwater tables in spring and the disturbance of the soil by slit injection of slurry increase topsoil desiccation. This decreases earthworm availability when it matters most for breeding meadow birds. Meadow bird conservation will benefit from revised manure application strategies that promote earthworm activity near or at the soil surface.

Global sensitivity analysis for identifying important parameters of nitrogen nitrification and denitrification under model uncertainty and scenario uncertainty

Nitrogen reactive transport modeling is subject to uncertainty in model parameters, structures, and scenarios. By using a new variance-based global sensitivity analysis method, this paper identifies important parameters for nitrogen reactive transport with simultaneous consideration of these three uncertainties. A combination of three scenarios of soil temperature and two scenarios of soil moisture creates a total of six scenarios. Four alternative models describing the effect of soil temperature and moisture content are used to evaluate the reduction functions used for calculating actual reaction rates. The results show that for nitrogen reactive transport problem, parameter importance varies substantially among different models and scenarios. Denitrification and nitrification process is sensitive to soil moisture content status rather than to the moisture function parameter. Nitrification process becomes more important at low moisture content and low temperature. However, the changing importance of nitrification activity with respect to temperature change highly relies on the selected model. Model-averaging is suggested to assess the nitrification (or denitrification) contribution by reducing the possible model error. Despite the introduction of biochemical heterogeneity or not, fairly consistent parameter importance rank is obtained in this study: optimal denitrification rate (Kden) is the most important parameter; reference temperature (Tr) is more important than temperature coefficient (Q10); empirical constant in moisture response function (m) is the least important one. Vertical distribution of soil moisture but not temperature plays predominant role controlling nitrogen reaction. This study provides insight into the nitrogen reactive transport modeling and demonstrates an effective strategy of selecting the important parameters when future temperature and soil moisture carry uncertainties or when modelers face with multiple ways of establishing nitrogen models.

Characteristics of soil moisture variation in different land use types in the hilly region of the Loess Plateau, China

Soil water availability is a key factor restricting the ecological construction and sustainable land use in the loess hilly region. It is of great theoretical and practical significance to understand the soil moisture status of different land use types for the vegetation restoration and the effective utilization of land resources in this area. In this study, EC-5 soil moisture sensors were used to continuously monitor the soil moisture content in the 0-160 cm soil profile in the slope cropland, terraced fields, jujube orchard, and grassland during the growing season (from May to October) in the Yuanzegou catchment on the Loess Plateau, to investigate soil moisture dynamics in these four typical land use types. The results showed that there were differences in seasonal variation, water storage characteristics, and vertical distribution of soil moisture under different land use types in both the normal precipitation (2014) and dry (2015) years. The terraced fields showed good water retention capacity in the dry year, with the average soil moisture content of 0-60 cm soil layer in the growing season being 2.6%, 4.2%, and 1.8% higher than that of the slope cropland, jujube orchard, and grassland (all P<0.05). The water storage of 0-160 cm soil profile was 43.90, 32.08, and 18.69 mm higher than that of slope cropland, jujube orchard, and grassland, respectively. In the normal precipitation year, the average soil moisture content of 0-60 cm soil layer in jujube orchard in the growing season was 2.9%, 3.8%, and 4.5% lower than that of slope cropland, terraced fields, and grassland, respectively (all P<0.05). In the dry year, the effective soil water storage of 0-160 cm soil profile in the jujube orchard accounted for 35.0% of the total soil water storage. The grey relational grade between the soil moisture in the surface layer (0-20 cm) and soil moisture in the middle layer (20-100 cm) under different land use types was large, and the trend for the similarity degree of soil moisture variation followed terraced fields > grassland > slope cropland > jujube orchard. The slope cropland in this area could be transformed into terraced fields to improve the utilization of precipitation and promote the construction of ecological agriculture. Aiming at resolving the severe water shortage in the rain-fed jujube orchard for the sustainable development of jujube orchard in the loess hilly region, appropriate water management measures should be taken to reduce the water consumption of jujube trees and other inefficient water consumption.

Nitrous Oxide Emissions from Soils with Different Vertical Soil Moisture Distribution Patterns

Nitrous Oxide Emissions from Soils with Different Vertical Soil Moisture Distribution Patterns

Diurnal pattern of nitrous oxide emissions from soils under different vertical moisture distribution conditions

The diurnal pattern of nitrous oxide (N2O) emissions is essential in understanding how weather and soil conditions influence the daily mean estimate of N2O fluxes. Incubation experiments were conducted to investigate the effects of vertical soil moisture distribution patterns on diurnal variation of N2O emissions. Clear diurnal patterns of N2O emissions on both surface watering (SW) and subsurface watering (SUW) treatments (SUW12, SUW15, and SUW18) were detected from soil sample (I), silty clay, and soil sample (II), sandy loam, where peak N2O fluxes usually occurred between 12:00 and 18:00 h. Different vertical watering patterns resulted in changes in the daily range of N2O fluxes and peak time. Mean fluxes from the SUW12, SUW15, and SUW18 treatments were 37.4%, 32.7%, and 43.3% lower than those from SW treatments from soil sample I, and 32.0%, 40.3%, and 41.1% from soil sample II. Moisture distribution patterns under SUW soils could be effective to mitigate N2O emissions. The N2O emissions from soil sample I ranged from178.3 to 2741.0 μg N2O m-2 h-1, which was more than in soil sample II with 7.0 to 83.7 μg N2O m-2 h-1. The different soil texture and N content level might account for the differences in magnitude of N2O fluxes from soils. The optimal soil moisture condition for peak N2O fluxes in the SW treatment had relatively narrower ranges than the SUW treatments with 46% to 60% water-filled pore space (WFPS) for soil sample I and 26% to 34% WFPS for soil sample II even though surface soil moisture for peak N2O fluxes were somewhat different from the previously reported optimal soil moisture range of 45% to 75% WFPS.

Vertical distribution of soil moisture and surface sandy soil wind erosion for different types of sand dune on the southeastern margin of the Mu Us Sandy Land, China.

Soil moisture is a critical state affecting a variety of land surface and subsurface processes. We report investigation results of the factors controlling vertical variation of soil moisture and sand transport rate of three types of dunes on the southeastern margin of the Mu Us Sandy Land. Samples were taken from holes drilled to a depth of 4 m at different topographic sites on the dunes, and were analyzed for soil moisture, grain-size distribution and surface sediment discharge. The results show that:(1) The average soil moisture varies in different types of dunes, with the following sequences ordered from highest to lowest: in the shrubs-covered dunes and the trees-covered dunes the sequence is from inter-dunes lowland to windward slope to leeward slope. The average moisture in the bare-migratory sand dunes is sequenced from inter-dunes lowland to leeward slope to windward slope.(2) Vegetation form and surface coverage affect the range of soil moisture of different types of dunes in the same topographic position. The coefficient of variation of soil moisture for shrubs-covered dunes is higher than that of other types of dune.(3) The effect of shrubs on dune soil moisture is explained in terms of the greater ability of shrubs to trap fine-grained atmospheric dust and hold moisture.(4) The estimated sand transport rates over sand dunes with sparse shrubs are less than those over bare-migratory dunes or sand dunes with sparse trees, indicating that shrubs are more effective in inhibiting wind erosion in the sandy land area.

Distribution of moisture and salinity under deficit irrigation and irrigation water salinity in an alternative trickle irrigation system of tape

This study was performed to investigate the horizontal and vertical distribution of soil moisture and salinity using an alternative trickle irrigation system of drip tape. Four main treatments consisting of 100, 80, 70, and 60% of the plants’ water requirements and three sub-treatments of 2.1, 4.6, and 10.2 dS/m, were conducted. Following irrigation, the soil moisture and salinity distribution around the emitters were measured every 24 h. The results showed that the accumulation of salts in the soil reduced the evaporation from the soil surface in treatments with high salinity. Therefore, in treatments with a low plant water requirement and high salinity levels, the volume of water in the soil is greater than in treatments with a high plant water requirement and low salinity levels. Although the crop yield is reduced with deficit irrigation, the saved water can be used to increase the area under cultivation, leading to increases in the overall crop yield.

High‐resolution ground‐penetrating radar monitoring of soil moisture dynamics: Field results, interpretation, and comparison with unsaturated flow model

Surface ground‐penetrating radar (GPR) techniques have been used by a number of previous researchers to characterize soil moisture content in the vadose zone. However, limited temporal sampling and low resolution near the surface in these studies greatly impedes the quantitative analysis of vertical soil moisture distribution and its associated dynamics within the shallow subsurface. To further examine the capacity of surface GPR, we have undertaken an extensive 26 month field study using concurrent high‐frequency (i.e., 900 MHz) reflection profiling and common‐midpoint (CMP) soundings to quantitatively monitor soil moisture distribution and dynamics within the shallow vadose zone. This unprecedented data set allowed us to assess the concurrent use of these techniques over two contrasting annual cycles of soil conditions. Reflection profiles provided high‐resolution traveltime data between four stratigraphic reflection events while cumulative results of the CMP sounding data set produced precise depth estimates for those reflecting interfaces, which were used to convert interval‐traveltime data into soil moisture. The downward propagation of major infiltration episodes associated with seasonal and transient events are well resolved by the GPR data. The use of CMP soundings permitted the determination of direct ground wave velocities, which provided high‐resolution information along the air‐soil interface. This improved resolution enabled better characterization of short‐duration wetting/drying and freezing/thawing processes, and permitted better evaluation of the nature of the coupling between shallow and deep moisture conditions. The nature of transient infiltration pulses, evapotranspiration episodes, and deep drainage patterns observed in the GPR data series were further examined by comparing them with a vertical soil moisture flow simulation based on the variably saturated model, HYDRUS‐1D. Using laboratory‐derived soil hydraulic property information from soil samples and a number of simplifying assumptions about the upper and lower‐boundary condition, we were able to achieve very good agreement between measured and simulated soil moisture profiles without model calibration; this is a strong indication of the overall quality of the GPR‐derived soil moisture estimates. The only notable difference between simulated values and GPR water content estimates occurred during extended dry soil conditions near the surface.

Gravity changes associated with variations in local land-water distributions: Observations and hydrological modeling at Isawa Fan, northern Japan

Gravity changes associated with variations in local land-water distributions have been observed at Isawa Fan in northern Japan, and modeled by hydrological equations. We solve the Richards equation numerically for the time variation in the vertical soil moisture distribution, which is then spatially integrated to estimate gravity changes due to the soil moisture distribution. In modeling Isawa Fan, we assume a simple hydrological model: a horizontally homogeneous soil in an infinite half-space. The estimated gravity is consistent with the observed gravity during a 50-day period within about 0.4 µgal root mean square, owing to both observed soil parameter values and the observation building geometry being incorporated into the hydrological model. However, the estimated gravity cannot fully reproduce annual gravity changes observed during a 2-year time frame, because the boundary conditions in the modeling determine only local water distributions and the resultant short-period gravity changes. Instead, the observed gravity over these 2 years can be reproduced within about 1.0 µgal root mean square, if the additional parameters of the annual gravity change (Aac and Aas) and the snowfall effect (As) are calculated by the function regression to the observed gravity with the least-squares method. The hydrological modeling techniques presented here can be utilized at all gravity sites in flat areas similar to Isawa Fan, such that hydrological effects in gravity data can be corrected and mass transfers associated with earthquakes and volcanoes can be monitored.