Combination Of Soil Moisture Research Articles

Smart Data Blending Framework to Enhance Precipitation Estimation through Interconnected Atmospheric, Satellite, and Surface Variables

Accurate precipitation estimation remains a challenge, though it is fundamental for most hydrological analyses. In this regard, this study aims to achieve two objectives. Firstly, we evaluate the performance of two precipitation products from the Integrated Multi-satellitE Retrievals for Global Precipitation Measurement (GPM-IMERG) for Sicily, Italy, from 2016 to 2020 by a set of categorical indicators and statistical indices. Analyses indicate the favorable performance of daily estimates, while half-hourly estimates exhibited poorer performance, revealing larger discrepancies between satellite and ground-based measurements at sub-hourly timescales. Secondly, we propose four multi-source merged models within Artificial Neural Network (ANN) and Multivariant Linear Regression (MLR) blending frameworks to seek potential improvement by exploiting different combinations of Soil Moisture (SM) measurements from the Soil Moisture Active Passive (SMAP) mission and atmospheric factor of Precipitable Water Vapor (PWV) estimations, from the Advanced Microwave Scanning Radiometer-2 (AMSR2). Spatial distribution maps of some diagnostic indices used to quantitatively evaluate the quality of models reveal the best performance of ANNs over the entire domain. Assessing variable sensitivity reveals the importance of IMERG satellite precipitation and PWV in non-linear models such as ANNs, which outperform the MLR modeling framework and individual IMERG products.

Soil Moisture Assimilation Improves Terrestrial Biosphere Model GPP Responses to Sub-Annual Drought at Continental Scale

Due to the substantial gross exchange fluxes with the atmosphere, the terrestrial carbon cycle plays a significant role in the global carbon budget. Drought commonly affects terrestrial carbon absorption negatively. Terrestrial biosphere models exhibit significant uncertainties in capturing the carbon flux response to drought, which have an impact on estimates of the global carbon budget. Through plant physiological processes, soil moisture tightly regulates the carbon cycle in the environment. Therefore, accurate observations of soil moisture may enhance the modeling of carbon fluxes in a model–data fusion framework. We employ the Carbon Cycle Data Assimilation System (CCDAS) to assimilate 36-year satellite-derived surface soil moisture observations in combination with flask samples of atmospheric CO2 concentrations. We find that, compared to the default model, the performance of optimized net ecosystem productivity (NEP) and gross primary productivity (GPP) has increased with the RMSEs reduced by 1.62 gC/m2/month and 10.84 gC/m2/month, which indicates the added value of the ESA-CCI soil moisture observations as a constraint on the terrestrial carbon cycle. Additionally, the combination of soil moisture and CO2 concentration in this study improves the representation of inter-annual variability of terrestrial carbon fluxes as well as the atmospheric CO2 growth rate. We thereby investigate the ability of the optimized GPP in responding to drought by comparing continentally aggregated GPP with the drought index. The assimilation of surface soil moisture has been shown to efficiently capture the influences of the sub-annual (≤9 months drought durations) and large-scale (e.g., regional to continental scales) droughts on GPP. This study highlights the significant potential of satellite soil moisture for constraining inter-annual models of the terrestrial biosphere’s carbon cycle and for illustrating how GPP responds to drought at a continental scale.

Increasing impact of compound agricultural drought and hot events on maize yield in China

Drought episodes with hot events usually trigger dramatic impacts on crop yields. However, most previous studies on climate stress have only focused on individual droughts or hot conditions, leading to a neglect of compound information. Based on various combinations of soil moisture and temperature, we investigated the impact of 6 modes of events, namely, compound drought and cold events, compound wet and hot events, compound agricultural drought and hot events (CADHEs), compound wet and cold events, droughts, and hot events, on maize yield in China. We present evidence from country-level, province-level, and grid-cell yield data that CADHEs have emerged as a major threat to maize yield, with higher yield reduction (as well as a higher probability of yield reduction) than the other 5 soil-moisture- and temperature-related events, including agricultural droughts defined only by soil moisture. The negative maize-yield anomalies due to CADHEs have increased during the past decades at both national and regional scales. The increase in the frequency, spatial extent, and severity of compound events can partly account for this enhanced negative impact on maize yield. The concurrent intensification in the positive soil-moisture-yield relationship and the negative temperature-yield relationship is likely responsible for this change. The negative response of maize yield suggests that if the increase in CADHEs persists, more adaptation and mitigation strategies are needed to ensure sustainable food security.

Effects of Soil Moisture and Temperature on Microbial Regulation of Methane Fluxes in a Poplar Plantation

Improved mechanistic understanding of soil methane (CH4) exchange responses to shifts in soil moisture and temperature in forest ecosystems is pivotal to reducing uncertainty in estimates of the soil-atmospheric CH4 budget under climate change. We investigated the mechanism behind the effects of soil moisture and temperature shifts on soil CH4 fluxes under laboratory conditions. Soils from the Huai River Basin in China, an area that experiences frequent hydrological shifts, were sampled from two consecutive depths (0–20 and 20–50 cm) and incubated for 2 weeks under different combinations of soil moisture and temperature. Soils from both depths showed an increase in soil moisture and temperature-dependent cumulative CH4 fluxes. CH4 production rates incubated in different moisture and temperature in surface soil ranged from 1.27 to 2.18 ng g−1 d−1, and that of subsurface soil ranged from 1.18 to 2.34 ng g−1 d−1. The Q10 range for soil CH4 efflux rates was 1.04–1.37. For surface soils, the relative abundance and diversity of methanotrophs decreased with moisture increase when incubated at 5 °C, while it increased with moisture increase when incubated at 15 and 30 °C. For subsurface soils, the relative abundance and diversity of methanotrophs in all samples decreased with moisture increase. However, there was no significant difference in the diversity of methanogens between the two soil depths, while the relative abundance of methanogens in both depths soils increased with temperature increase when incubated at 150% water-filled pore space (WFPS). Microbial community composition exhibited large variations in post incubation samples except for one treatment based on the surface soils incubated at 15 °C, which showed a decrease in the total and unique species number of methanotrophs with moisture increase. In contrast, the unique species number of methanogens in surface soils increased with moisture increase. The analysis of distance-based redundancy analysis (db-RDA) showed that soil pH, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), microbial biomass carbon (MBC), NO3−-N, and NH4+-N mainly performed a significant effect on methanotrophs community composition when incubated at 60% WFPS, while they performed a significant effect on methanogens community composition when incubated at 150% WFPS. Overall, our findings emphasized the vital function of soil hydrology in triggering CH4 efflux from subtropical plantation forest soils under future climate change.

Effect of root-zone vertical soil moisture heterogeneity on water transport safety in soil-plant-atmosphere continuum in Robinia pseudoacacia

Soil moisture in root zone is highly heterogeneous in space, while its effect on water transport safety in soil-plant-atmosphere continuum (SPAC) remains poorly understood. In this study, we conducted vertical spilt-root experiments in R. pseudoacacia using loamy clay and sandy loam soils in greenhouse, and measured the dynamics of midday transpiration rate, predawn and midday leaf water potential with the lower root zone remaining drought and the upper root zone undergoing the drought-rewatered-drought process. The plant supply-demand hydraulic model was calibrated with the measured data, indicating that the model could efficiently simulate SPAC water transport in R. pseudoacacia under the condition of vertical soil moisture heterogeneity. On this basis, we set various combinations of soil moisture in the upper and lower root zones under different soil types and atmospheric evaporative demands in the model, and simulated the variations of indicators describing water transport safety in SPAC, including actual transpiration rate (E), the critical leaf transpiration rate at hydraulic failure (Ecrit), hydraulic safety margin (HSM), and percentage loss of soil-plant hydraulic conductance (PLK). The numerical simulations suggested that the water transport safety in SPAC varied substantially with vertical soil moisture heterogeneity, and the responses were impacted by soil types and atmospheric evaporative demand. With decreasing soil moisture in the upper root zone (SMCup), Ecrit, E and HSM remained steady at first and then decreased rapidly when SMCup below a threshold, while PLK exhibited an opposite trend. With decreasing soil moisture in the lower root-zone (SMCdown), the curves of Ecrit, E and HSM presented a descending trend, while the curve of PLK went up. Water transport safety in SPAC declined with decreasing SWCup and SWCdown and became more sensitive to SWCup with a lower SWCdown. SWCdown had greater impact on water transport safety in SPAC under coarser-textured soil with higher atmospheric evaporative demand. The results were supplemental to the traditional analysis of soil water availability to plants under homogeneous condition, and would be helpful for analyzing functional difference of water storage in different soil depths as well as for optimizing water resource management.

Assimilation of near-real time data products into models of an urban basin

The goal of this study was to determine if assimilating a combination of various derived data products can help circumvent some of the difficulties associated with urban watershed modeling. Combinations of the SNODAS (Snow Data Assimilation System) snow water equivalent data, the SMOS (Soil Moisture and Ocean Salinity) L2 soil moisture, and streamflow observations were used for the data assimilation schemes. Combinations of these observation data sets were assimilated into lumped conceptual rainfall-runoff models of the highly-urbanized Don River basin (in southern Ontario) to determine if assimilation of geophysical variables will have a significant impact on simulations and forecasting in an urbanized watershed. The Ensemble Kalman Filter (EnKF) data assimilation method was used for these analyses, with various rainfall-runoff models that include GR4J, HyMod, MAC-HBV, and SAC-SMA models. The best data assimilation scheme for hydrologic modeling involved using a combination of streamflow, soil moisture, and snow water equivalent while performing both state and parameter updating. These results suggest that using a combination of soil moisture and snow water equivalent from the SMOS and SNODAS data products can improve simulations and ensemble forecasts in an urban basin.

Agronomic evaluation of seedbed and mulching alternatives with plant spacing for dry‐season fluted pumpkin in coarse‐textured tropical soil

AbstractFluted pumpkin (Telfairia occidentalis) has high agronomic and food relevance in the tropics. Production of this vegetable in the dry season with irrigation is increasingly popular and is mostly on droughty soils, thus requiring research on appropriate combination of soil moisture conservation/management practices. This study evaluated the interactions among seedbed configuration (flat and raised), surface mulching (grass‐mulched and no‐mulch) and plant spacing (1002, 502 and 252 cm2, designated wide, intermediate and close spacing, respectively) on fluted pumpkin performance in a sandy‐loam soil in southeastern Nigeria during 2014/2015 dry season. Seedling emergence (ratio of emerged to expected seedlings) was highest and lowest in flat/mulched/wide and flat/no‐mulch/close combinations, respectively. Main effects showed higher seedling emergence in mulched (67.9%) than no‐mulch plots (53.1%) and in wide (70.8%) and intermediate spacing (64.6%) compared to close spacing (46.0%). Fresh leaf yield assessed biweekly during 6–14 weeks after sowing was generally highest and lowest in flat/mulched/close and flat/no‐mulch/wide combinations, respectively. Seedbed configuration had no distinct effect on leaf yield; hence, flat/mulched/close option was similar to raised/mulched/close option. Yields from mulched/wide and no‐mulch/close options were similar with a mean (2735 kg/ha) approximating the overall mean (2868 kg/ha). Flat/mulched/close combination is the best management practice to grow fluted pumpkin with irrigation on droughty soils during the dry season of the tropics. Where this best practice is compromised due to mulch or seed scarcity, the ensuing no‐mulch/close and mulched/wide, respectively can support dry‐season production of pumpkin in droughty tropical soils at average levels.

Cross-biome assessment of gross soil nitrogen cycling in California ecosystems

Microbial transformations of nitrogen (N) largely determine whether N is retained in ecosystems via net primary productivity or lost via gaseous emissions and leaching. The controls on soil N cycling are often studied at single locales, making it difficult to predict N cycling at regional to global scales. We hypothesized that contemporary soil properties exhibit consistent relationships with instantaneous gross N cycling rates across diverse biomes that create a continuum in these properties. We measured ex situ gross N cycling rates and soil properties at 33 study sites representing five biome classifications in California including deserts, grasslands, shrublands, forest, and wetlands. Desert soils had significantly lower total N, organic carbon (C), microbial biomass N, and soil moisture as well as higher pH than all other biomes, whereas forests and wetlands had significantly lower soil nitrate (NO3−) concentrations (P < 0.001 for all). Gross mineralization rates were best predicted by the combination of soil moisture and soil C:N ratios (R2 = 0.46), which exerted positive and negative controls, respectively. Grasslands exhibited marginally higher gross mineralization than all other biomes, whereas deserts had the lowest rates due to low soil moisture (P = 0.09). Gross nitrification rates were positively correlated to soil NO3− concentrations (R2 = 0.34) and negatively correlated to soil C:N ratios (R2 = 0.31). The negative relationship between gross nitrification and soil C:N ratios was driven by forest soils, which had significantly higher C:N ratios and lower gross nitrification than all other biomes (P < 0.05). Dissimilatory NO3− reduction to NH4+ (DNRA) occurred in soils from all biomes. The strong positive correlation between DNRA rates and soil NO3− (R2 = 0.41) suggests NO3− limitation of DNRA. Predictable patterns in gross N cycling across biomes in California suggest that contemporary soil properties are important drivers of instantaneous soil N cycling rates that integrate over differences in vegetation type, atmospheric N deposition rates, and local climate.

Multiple timescale variations and controls of soil respiration in a tropical dry dipterocarp forest, western Thailand

This study aims to improve our understanding of temporal variations and controlling factors of soil respiration (Rs) and its components (microbial respiration or Rm and root respiration or Rb) in an Asian tropical seasonal forest at diurnal, seasonal and annual timescales in relation to biotic and abiotic controls. Rs was studied by the trenching method in a seasonal dry diptercarp forest, western Thailand. An automated soil chamber system was used to produce hourly data of Rs during 2008–2011. Analysis of Rs in relation to both biotic and abiotic factors was carried out to understand its temporal variations at different timescales. Rm was the main contributor to overall magnitude and variability of Rs. Soil temperature alone was the main driver of diurnal variation, while the combination of soil moisture and soil temperature determined the seasonal variations. The amount of Rs was also related to the fine root (<2 mm) and microbial biomass at seasonal timescales. Due to the small inter-annual variations in soil temperature and moisture, total soil respiration among the 4 years was not significantly different (p < 0.05). The annual totals for Rs during 2008–2011 were 3.20, 3.89, 3.52, 4.14 kgCO2 m−2 years−1, respectively. The 4-year average ratio of Rm (trenched) to Rs (untrenched) was 66 ± 4 %. Rm plays a crucial role in determining the magnitude (large ratio between Rm and Rs) and temporal variations of Rs. In this forest ecosystem, high seasonal variations in Rs were observed and were mainly attributed to the response of Rm to moisture.

Effects of Soil Rhizosphere Aeration on the Root Growth and Water Absorption of Tomato

AbstractSoil rhizosphere aeration status is an important aspect of soil quality and soil ecology. The objective of the current study was to determine the appropriate moisture environment that facilitates rhizosphere soil aeration and ensures normal root respiration in tomato. In the potted experiment, five treatments of soil aeration were used (0.4, 0.8, 1.2, 1.6 ventilation volume of 50% porosity of soil, and no ventilation) under conditions of the different soil moisture upper limits. The effects of different rhizosphere soil aerations on the physiological indicators and water absorption of tomato were studied. Under the same soil moisture condition, plant growth and root vitality initially increased, and then decreased when the soil ventilation volume increased. The combination of soil moisture with 80% of field capacity and 0.8 ventilation volume with 50% soil porosity raised the chlorophyll content by 29.98% and the root vitality by 61.55%, as compared with the non‐ventilated treatment. Therefore, the appropriate volume of rhizosphere ventilation can effectively improve the capacity of water absorption in tomato. The result provides a new view about soil quality and soil ecology in terms of soil–root system.

Effects of moisture conditions on potential soil water repellency in a tropical forest regenerated after fire

Potential water repellency (PWR) is a common index to indicate the degree to which soils repel water. Soil organic matter (SOM) is a requisite substance for the water repellency but many studies have shown that SOM content alone could not fully account for the observed variation in PWR. We investigated potential factors responsible for PWR of soils in a tropical forest in East Kalimantan, Indonesia. In addition to the well-investigated soil properties (e.g., total SOM content, pH, and the amount of iron or aluminum oxides), we also focused on soil moisture content at the time of sampling and water-extractable organic matter (WEOM) content based on the hypothesis that PWR may depend on amphiphilic fractions, including WEOM, in the outermost layer of SOM adsorbed on soil particles — soil water may change the amount and/or the conformation of the amphiphilic fractions. Results showed that the degree of PWR had the highest correlation with the amount of WEOM, not with SOM, among the factors investigated (R2=0.29). The WEOM content (R2=0.65) better explained the variation in water repellency than SOM content (R2=0.47) even after soils were soaked in n-hexane (a non-polar solvent) and expected to have uniform SOM conformation (hydrophobic components dominant on the surfaces). The combination of soil moisture and SOM contents better explained the PWR than SOM content alone. These results suggest that soil water content can have substantial effects on PWR by changing the availability and/or conformation of the amphiphilic SOM, including WEOM.

Effect of Soil Moisture Conserving Irrigation under Straw Mulching on Soil and Crop

Currently, the agricultural water shortage in China has become more and more serious. It is of great importance for scientists and engineers to figure out the ways to utilize the limited water resources efficiently for irrigation. This paper proposes a new irrigation concept which combines advanced soil moisture conserving techniques available in the semi-arid and arid regions of China with the design of water-saving irrigation systems. To show the effectiveness of the new concept, straw mulching used widely in China, a soil moisture conserving technique, was tested in irrigation systems. The experimental results show the way soil moisture, soil microorganism, soil temperature, ecological and physiological indices and maize productivity were affected by straw mulching under irrigation and the improved performance indices with the combination of soil moisture conserving techniques with irrigation systems, which contributed to a higher water use efficiency. It is believed that the combination of soil moisture conserving techniques with irrigation systems will be a new trial for the water-saving irrigation of China, particularly for the semi-arid and arid areas.

Effects of soil moisture and temperature on CO 2 and CH 4 soil–atmosphere exchange of various land use/cover types in a semi-arid grassland in Inner Mongolia, China

The aim of this study was to investigate the combined effects of soil moisture and temperature as well as drying/re-wetting and freezing/thawing on soil–atmosphere exchange of CO 2 and CH 4 of the four dominant land use/cover types (typical steppe, TS; sand dune, SD; mountain meadow, MM; marshland, ML) in the Xilin River catchment, China. For this purpose, intact soil cores were incubated in the laboratory under varying soil moisture and temperature levels according to field conditions in the Xilin River catchment. CO 2 and CH 4 fluxes were determined approximately daily, while soil CH 4 gas profile measurements at four soil depths (5 cm, 10 cm, 20 cm and 30 cm) were measured at least twice per week. Land use/cover generally had a substantial influence on CO 2 and CH 4 fluxes, with the order of CH 4 uptake and CO 2 emission rates of the different land use/cover types being TS ≥ MM ≥ SD > ML and MM > TS ≥ SD > ML, respectively. Significant negative soil moisture and positive temperature effects on CH 4 uptake were found for most soils, except for ML soils. As for CO 2 flux, both significant positive soil moisture and temperature effects were observed for all the soils. The combination of soil moisture and temperature could explain a large part of the variation in CO 2 (up to 87%) and CH 4 (up to 68%) fluxes for most soils. Drying/re-wetting showed a pronounced stimulation of CO 2 emissions for all the soils —with maximum fluxes of 28.4 ± 2.6, 50.0 ± 5.7, 81.9 ± 2.7 and 10.6 ± 1.2 mg C m −2 h −1 for TS, SD, MM and ML soils, respectively—but had a negligible effect on CH 4 fluxes (TS: −3.6 ± 0.2; SD: 1.0 ± 0.9; MM: −4.1 ± 1.3; ML: −5.6 ± 0.8; all fluxes in μg C m −2 h −1). Enhanced CO 2 emission and CH 4 oxidation were observed for all soils during thawing periods. In addition, a very distinct vertical gradient of soil air CH 4 concentrations was observed for all land use/cover types, with gradually decreasing CH 4 concentrations down to 30 cm soil depth. The changes in soil air CH 4 concentration gradients were in accordance with the changes of CH 4 fluxes during the entire incubation experiment for all soils.

Interannual variations of river water storage from a multiple satellite approach: A case study for the Rio Negro River basin

Spatiotemporal variations of water volume over inundated areas located in a large river basin have been determined using combined observations from a multisatellite inundation data set, the TOPEX/POSEIDON (T/P) altimetry satellite, and in situ hydrographic stations for the water levels over rivers and floodplains. We computed maps of monthly surface water volume change over the period of common availability of T/P and the multisatellite data (1993–2000). The basin of the Negro River, the largest tributary in terms of discharge to the Amazon River, was selected as a test site. A strong seasonal signal is observed with minima in October and maxima in June. A strong interannual component is also present, particularly important during ENSO years. The surface water change was estimated to be 167 ± 39 km3 between October 1995 (low water) and June 1996 (high water). This result is consistent with previous estimates obtained for the 1995–1996 hydrological cycle over the same area using the JERS mosaic data. The surface water volume change is then compared to the total water volume change inferred from the GRACE satellite for an average annual cycle. The difference between the surface storage change and the total storage change derived from GRACE was computed to estimate the contribution of the soil moisture and groundwater to the total storage change. Our study supports the hypothesis that total water storage is almost equally partitioned between surface water and the combination of soil moisture and groundwater for the Negro River basin. The water volume changes are also evaluated using in situ discharge measurements and the GPCP precipitation product (correlation of 0.61). The results show the high potential for the new technique to provide valuable information to improve our understanding of large river basin hydrologic processes.

Habitat Factors Influencing Distributions ofAnaplasma phagocytophilumandEhrlichia chaffeensisin the Mississippi Alluvial Valley

Human monocytotropic ehrlichiosis (HME), caused by the bacterium Ehrlichia chaffeensis, and human granulocytic anaplasmosis (HGA), caused by the bacterium Anaplasma phagocytophilum, are two emerging tick-borne zoonoses of concern. Factors influencing geographic distributions of these pathogens are not fully understood, especially at varying spatial extents (regional versus landscape) and resolutions (counties versus smaller land units). We used logistic regression to compare influences of physical environment, land cover composition, and landscape heterogeneity on distributions of A. phagocytophilum and E. chaffeensis at multiple spatial extents. Pathogen presence or absence was determined from white-tailed deer (Odocoileus virginianus) serum samples collected from 1981 to 2005. Ecological predictor variables were derived from spatial datasets that represented deer density, elevation, land cover, normalized difference vegetation index (NDVI), hydrology, and soil moisture. We used three strategies (a priori, exploratory, and spatial extent) to develop models. Best fitting models were applied within a geographic information system to create predictive probability surfaces for each bacterium. Ecological predictor variables generally resulted in better fitting models for E. chaffeensis than A. phagocytophilum (90.5% and 68% sensitivity, respectively), possibly as a result of differences in the natural histories of tick vectors. Although alternative model development strategies produced different models, in all cases bacteria presence or absence was affected by a combination of soil moisture or flooding variables (thought to affect primarily tick vectors) and forest cover or NDVI variables (thought to affect primarily mammalian hosts). This research demonstrates the potential for modeling the distributions of microscopic tick-borne pathogens using coarse regional datasets and emphasizes the importance of forest cover and flooding as environmental constraints, as well as the importance of considering ecological variables at multiple spatial extents.

Temporal and spatial variability in root reinforcement of streambanks: Accounting for soil shear strength and moisture

Riparian vegetation exerts a number of mechanical and hydrologic controls on bank stability, which can affect the delivery of sediment to channels. Estimates of root reinforcement of soils have commonly been attained using perpendicular root models that simply sum root tensile strengths and consider these as an add-on factor to soil strength. A major limitation of such perpendicular models is that tensile strength and resistance is wrongly considered to be independent of soil type and moisture, and therefore variations according to these bank properties are omitted in conventional models. In reality, during mass failure of a streambank, some roots break, and some roots are pulled out of the soil intact; the relative proportions of roots that break or pull out are determined by a combination of soil moisture and shear strength. In this paper an equation to predict the frictional resistance of root–soil bonds was tested against field data collected at Long Creek, MS, under two soil moisture conditions. The root pullout equations were then included in the root-reinforcement model, RipRoot, and bank stability model runs for Goodwin Creek, MS, were carried out in order to examine the effects of spatial and temporal variations in soil shear strength and rooting density, on streambank factor of safety. Model results showed that at smaller root diameters breaking forces exceeded pullout forces, but at larger root diameters pullout forces exceed breaking forces. The threshold diameter between root pullout and root breaking varied with soil shear strength, with increasing soil shear strength leading to a greater proportion of roots failing by breaking instead of pullout. Root-reinforcement estimates were shown to reflect changes in soil shear strength, for example, brought about by variations in soil matric suction. Resulting Factor of safety ( F S) values for the bank during the period modeled ranged from 1.36 to 1.74 with 1000 grass roots/m 2, compared to a range of 0.97 to 1.37 for the non-vegetated bank. Root reinforcement was shown to increase bank stability under the entire range of soil moisture conditions modeled. However, the magnitude of root reinforcement varied in both space and time as determined by soil shear strength and soil moisture.

SCOUR BELOW AN OVERFALL: PART I. INVESTIGATION

Scour below an overfall contributes to headcut instability and gully advance. Thirteen largescale scour tests wereconducted by preforming an overfall in a compacted cohesive soil bed. Horizontally compacted soil layers were placed usingtypical embankment construction methods. Water flowing over a horizontal approach was allowed to plunge over an overfalland impinge on the compacted soil bed. Different combinations of soil moisture and soil density were examined for two similartailwater or backwater conditions. For a constant flow rate, the vertical scour was measured downstream of the gully overfall,and typical erosion behavior is discussed. The rate of vertical scour was observed as the plunge pool deepened.<br><br>The vertical scour rate increased as the moisture content at the time of compaction decreased. The scour rate was alsoobserved to decrease as the dry density of the fill material increased. For the soil conditions tested, the vertical scour rateincreased as the unconfined compressive strength increased. The stressstrain modulus and the vertical scour rate alsodisplayed a direct relationship. The layered soil placement methods, typical of embankment construction, can have aninfluence on the observed vertical scour rates. These fieldscale erosion tests were conducted to enhance our understandingof scour processes. This scour investigation focuses on the test description, observed results, and fundamental relationships,while a companion article addresses prediction methods.

Rainfall-Runoff Relation for Three Mountainous Arid Basins

Regression analysis was utilized to investigate the rainfall- runoff relations of three mountainous basins in the western and southwestern regions of Saudi Arabia. Areal mean rainfall depth was computed using Thiessen polygons method for different combination of rainfall stations to investigate which one produced the best result. Regression analysis was performed for four different combinations of soil moisture conditions and rainfall locations. Spatial distribution of rainfall is found to be the most important factor affecting values of runoff coefficients for the study basins.

Nitric oxide emission from a Norway spruce forest floor

Although the available data on soil NO emission are increasing, only few measurements have been made in temperate forests. Emissions from these forests are generally low, but recent studies have shown that NO emission from N‐affected forests can be much higher. In this study the emission of NO from the floor of a moderately N‐affected Norway spruce forest in Denmark was measured using dynamic chambers during two seasons (April 1995 and September 1995). At the same time, fluxes of CO2 and NO2, soil water content inside and outside the chambers, soil temperature at 10 cm depth, soil surface temperature and air temperature were recorded. The emission of NO in April was low (&lt; 0.3 ng N m−2 s−1), whereas it was significantly higher in September (21 ng N m−2 s−1 with a range 0–82 ng N m−2 s−1). In April the soil surface temperature was low (&lt; 6°C) and the soil moisture was high (26% vol/vol). In September the soil surface temperature was high (16°C) and soil moisture was significantly lower (14% vol/vol) than in April. There was a high correlation between CO2 emission and NO emission. The fluxes of NO2 were very small and not related to NO emission. Generally, the variation within a single plot was small, whereas the variation between plots was large. About half of the total variation of NO emission in September could be explained by a combination of soil moisture, distance to tree trunks, and moss cover. No significant relationship could be found between NO emission and soil surface temperature or soil temperature. Other parameters such as the C/N dynamics in the soil must be taken into account to provide a better mechanistic understanding of the NO emission. An annual emission of 3.15 kg NO‐N ha−1 yr−1 was estimated by the amount of NH4+ in throughfall. Compared to most of the previous published measurements of NO emission from temperate forest soils, the values presented here are high and of the same magnitude as found in N‐affected forests in Central Europe.

Temporal variation in temperature and rainfall differentially affects ectomycorrhizal colonization at two contrasting sites

We examined the roles that seasonal shifts in precipitation and temperature played in the ectomycorrhizal (ECM) colonization of pinyon pine (Pinus edulis Engelm.) at two contrasting sites in northern Arizona. Pinyons growing in ash and cinder soils experienced much greater water and nutrient stress than pinyons growing nearby in sandy‐loam soils. Over a one year period, we obtained monthly measurements of ECM colonization, root zone soil moisture and temperature, and air temperature and precipitation. Four major patterns emerged. Firstly, although climate as measured by ambient temperature and precipitation did not vary between the two sites, soil temperature was significantly higher and soil moisture significantly lower at the cinder site than at the sandy‐loam site. Secondly, ECM colonization was significantly higher at the cinder site for 5 of 12 months. Thirdly, although nearly 70% of the variation in ECM colonization of pinyons growing in cinder soil was predicted by a combination of soil moisture and soil temperature, these same variables had little predictive power for pinyons growing in sandy‐loam soils. Air temperature and precipitation were also significantly correlated with ECM colonization at the cinder site but not the sandy‐loam site. Fourthly, a watering experiment showed that ECM colonization significantly increased with supplemental water at the cinder site, but not at the sandy‐loam site. Thus, in two sites that did not differ in plant community or climate, ectomycorrhizas in cinder soils were far more sensitive to changes in moisture and temperature than ectomycorrhizas in sandy‐loam soils.