Root-zone Water Storage Research Articles

Revisiting the definition of field capacity as a functional parameter in a layered agronomic soil profile beneath irrigated maize

The soil water content at the condition of field capacity (θFC) is a key parameter in irrigation scheduling and has been suggested to be determined by running a synthetic drainage experiment until the flux rate (q) at the bottom of the soil profile achieves a predefined negligible value (qFC). We question the impact of qFC on the assessment of field capacity. Moreover, calculating θFC as the integral mean of the water content profile when q is equal to qFC is strictly valid only for uniform soil profiles. By contrast, this practice is ambiguous and biased for stratified soil profiles due to the soil water content discontinuity at the layer interfaces. In this study, the concept of field capacity was revisited and adapted to practical agronomic heuristics. By resorting to the assessment of root-zone water storage capacity (W), we envision field capacity as a functional hydraulic parameter derived from synthetic irrigation scheduling scenarios to minimize drought stress, drainage, and nitrate leachate below the root zone. A functional analysis was carried out on a 135-cm-thick layered soil profile beneath maize in eastern Nebraska. On-farm irrigation scheduling applications and agricultural practices were recorded for 20 years (2001–2020) at a daily time step. Hydrus-1D was calibrated and validated with direct measurements of the soil water retention curve and soil water content data, respectively, in each soil layer. A set of functional field capacity values was derived from 24 irrigation scheduling scenarios, and the optimal water storage capacity at field capacity (WFC) was approximately 50 cm (corresponding to about 80% saturation in the soil profile). An average irrigation amount of 217.5 mm distributed over 21 events was obtained by using optimal irrigation scheduling, which was initiated when the matric pressure head took on a value of −700 cm and the irrigation rate was set at 1.0 cm d−1. This irrigation practice ensured water storage at approximately the same level (ideally at WFC) by sustaining only evapotranspiration fluxes in the uppermost portion of the root zone and by limiting excessive drainage. This protocol can be transferred to other agricultural fields.

Contrasting ecosystem vegetation response in global drylands under drying and wetting conditions.

Increasing aridity is one major consequence of ongoing global climate change and is expected to cause widespread changes in key ecosystem attributes, functions, and dynamics. This is especially the case in naturally vulnerable ecosystems, such as drylands. While we have an overall understanding of past aridity trends, the linkage between temporal dynamics in aridity and dryland ecosystem responses remain largely unknown. Here, we examined recent trends in aridity over the past two decades within global drylands as a basis for exploring the response of ecosystem state variables associated with land and atmosphere processes (e.g., vegetation cover, vegetation functioning, soil water availability, land cover, burned area, and vapor-pressure deficit) to these trends. We identified five clusters, characterizing spatiotemporal patterns in aridity between 2000 and 2020. Overall, we observe that 44.5% of all areas are getting dryer, 31.6% getting wetter, and 23.8% have no trends in aridity. Our results show strongest correlations between trends in ecosystem state variables and aridity in clusters with increasing aridity, which matches expectations of systemic acclimatization of the ecosystem to a reduction in water availability/water stress. Trends in vegetation (expressed by leaf area index [LAI]) are affected differently by potential driving factors (e.g., environmental, and climatic factors, soil properties, and population density) in areas experiencing water-related stress as compared to areas not exposed to water-related stress. Canopy height for example, has a positive impact on trends in LAI when the system is stressed but does not impact the trends in non-stressed systems. Conversely, opposite relationships were found for soil parameters such as root-zone water storage capacity and organic carbon density. How potential driving factors impact dryland vegetation differently depending on water-related stress (or no stress)is important, for example within management strategies to maintain and restore dryland vegetation.

Water Age Dynamics in Plant Transpiration: The Effects of Climate Patterns and Rooting Depth

AbstractWater ages in plant transpiration reveal important processes of water dynamics in soil‐plant‐atmosphere continuum, which is regulated by water variability and availability in both precipitation inputs and root zone. However, the impacts of climate patterns and rooting depth on water age dynamics in transpiration are inadequately investigated. In this study, variations in root‐zone water replenishment are estimated based on stable isotope compositions of precipitation and plant xylem water, and a piecewise linear mixing water age model is employed to quantify water ages in plant transpiration under five different climates. The results show that water ages in transpiration are very dynamic in all climates with a range between 1 and 229 days. Water replenishments in the root‐zone reveal whether and how much precipitation enters the root zone. Climate patterns control median (mean) water age in plant transpiration but modified by rooting depth. In wet climates, plants mainly rely on water source that originates from precipitation in the current month (accounting for about 60% on average of their water sources), while plants in dry climates largely rely on water source that originates from precipitation fallen on previous months/seasons. These indicate that climate has profound impacts on plant water source by regulating precipitation inputs, root zone water storage and residence time, and evapotranspiration. Despite the limitations of this study, our study provides an alternative way to reveal the precipitation partitioning in root‐zone and seasonal changes in plant water use of precipitation, which highlights different vegetation responses to water variability between climate types.

Widespread root-zone water bypass for various climates and species: Implications for the ecohydrological separation understanding

Root-zone water storage is a core component of many hydrological systems, and a critical variable influencing water transport in the soil-vegetation-atmosphere continuum and biogeochemical processes. However, the dynamics of water bypass and replenishment in root-zone during precipitation infiltration processes have been rarely investigated. Here we employed a piecewise isotope balance method to estimate the proportion of root-zone water replenished by precipitation, and conducted a meta-analysis to check the patterns of water bypass and replenishment in root-zone for various species across five climate zones and examined the potential influencing factors. The results show that high proportion of root-zone water replenishment is likely to appear during the transition period from dry season/soil to wet season/soil. Root-zone water bypass is widespread under various climates, which is likely to occur by downward drainage under wet soil condition or via evaporation loss under high atmospheric demand. Antecedent soil water condition and rooting depth are found to be the two key factors regulating dynamics of root-zone water bypass and replenishment. These findings provide useful insight into our understanding of ecohydrological modeling (e.g., non-well-mixed water pool in soils) and plant responses to precipitation pulses.

Tillage timing to improve soil water storage in Mediterranean long fallow

Creation of a dry soil mulch to maintain near-surface moisture in dry environments is an ancient practice that is still used in many regions of the world for timely establishment of crops after long fallow. Before availability of herbicides, the first tillage during fallow was timed to kill weeds, but now the optimum timing for tillage depends only on the interplay of evaporation and infiltration of rain. The goal is to maximize total root zone water storage and seed-zone water at seeding time. Our objective was to measure the effect of delaying soil mulch creation progressively later in the spring and summer on weed-free fallow for winter wheat (Triticum aestivum L.) in the Mediterranean inland Pacific Northwest drylands of the United States. Over a period of six years and a total of 12 site-years, four to seven tillage timings were compared to an untilled treatment. Soil water was measured at seeding time in the seed zone and total root zone. Seed-zone water and total stored water produced by different tillage timings was surprisingly constant, with few statistically significant or substantial differences. On average, only 2% of stored water was lost in the top 1.0 m of soil over 80 days during the hot, dry period towards the end of fallow. Responses to late spring rain could be seen, and tillage near the end of June (early summer) often produced maximum soil water content.

Response and contribution of shallow groundwater to soil water/salt budget and crop growth in layered soils

Shallow groundwater affects crop growth and yield by altering the root zone water and salt budgets, with the process being more complicated in layered soils. To gain insights into the role of shallow groundwater in layered soils, LAWSTAC, a physically-based agro-hydrological model, was employed to simulate soil water, salt, and groundwater fluxes and crop yield in three soil profiles: loam (L), loam with a sand interlayer (LSL), and loam with a clay interlayer (LCL). The model was verified using data collected in 2007 and 2008 from a spring wheat field with shallow groundwater tables. The model predictions matched well with the measured soil water content, salt concentration, groundwater table depth (GWD), leaf area index, biomass, and crop yield. Further simulations were conducted for the L, LSL, and LCL profiles with varying GWDs in the crop growth seasons and initial values specified at 130, 150, 170, 190, 210, 230, and 250 cm. The groundwater table showed greater fluctuations in LCL, with smaller fluctuations in LSL in comparison to L. When starting with smaller initial GWDs (of 130, 150, 170, and 190 cm), the coarse-textured soil under the root zone facilitated water and salt fluxes at the bottom of the root zone, resulting in larger averages for root zone water storage and more salt content during the crop growth seasons and lower crop yield due to salt stress. Meanwhile, in cases with an initial GWD larger than 210 cm, the coarse-textured soil inhibited the fluxes and improved crop yield due to the reduced salt stress. Fine-textured soil under the root zone inhibited water and salt fluxes, reducing root zone salt content and favoring crop yield; however, the effect decreased with larger initial GWDs. Thus, the groundwater table should be lowered to a level where coarse-textured soil acts as a capillary barrier for salt control and crop growth in soil profiles similar to LSL. Keeping a high groundwater table to sustain water for crop growth is acceptable in soil profiles similar to LCL, while keeping a lower groundwater table could be an effective practice to control salt and improve crop production in salinized croplands.

A satellite-based approach to estimating spatially distributed groundwater recharge rates in a tropical wet sedimentary region despite cloudy conditions

Groundwater recharge (GWR) is one of the most challenging water fluxes to estimate, as it relies on observed data that are often limited in many developing countries. This study developed an innovative water budget method using satellite products for estimating the spatially distributed GWR at monthly and annual scales in tropical wet sedimentary regions despite cloudy conditions. The distinctive features proposed in this study include the capacity to address 1) evapotranspiration estimations in tropical wet regions frequently overlaid by substantial cloud cover; and 2) seasonal root-zone water storage estimations in sedimentary regions prone to monthly variations. The method also utilises satellite-based information of the precipitation and surface runoff. The GWR was estimated and validated for the hydrologically contrasting years 2016 and 2017 over a tropical wet sedimentary region located in North-eastern Brazil, which has substantial potential for groundwater abstraction. This study showed that applying a cloud-cleaning procedure based on monthly compositions of biophysical data enables the production of a reasonable proxy for evapotranspiration able to estimate groundwater by the water budget method. The resulting GWR rates were 219 (2016) and 302 (2017) mm yr−1, showing good correlations (CC = 0.68 to 0.83) and slight underestimations (PBIAS = −13 to −9%) when compared with the referenced estimates obtained by the water table fluctuation method for 23 monitoring wells. Sensitivity analysis shows that water storage changes account for +19% to −22% of our monthly evaluation. The satellite-based approach consistently demonstrated that the consideration of cloud-cleaned evapotranspiration and root-zone soil water storage changes are essential for a proper estimation of spatially distributed GWR in tropical wet sedimentary regions because of their weather seasonality and cloudy conditions.

Influence of coarse‐textured soil layers under crop root zone on soil water and salt dynamics and crop yield in shallow groundwater areas

AbstractCoarse‐textured soil layers under crop root zone could modify soil water and salt dynamics and affect crop growth. However, the influence is not thoroughly investigated, especially under shallow groundwater table and soil salinization conditions. For this purpose, we calibrated and validated a one‐dimensional agro‐hydrological model, LAWSTAC, for soil water and salt movement, and their interactions with maize growth in Northwest China. Simulation results fitted well with the measured values of soil water content, salt concentration, leaf area index, and crop yield for the maize growing seasons of 2012 and 2013. Further simulations were conducted to evaluate the influence of buried depths (80, 90, 100, and 110 cm) and thicknesses (10, 20, 30, 40, and 50 cm) of a coarse sand layer under root zone on soil water storage, salt leaching, and crop yield. Results demonstrated that soil water and salt status in the root zone was related to the water content of the coarse‐textured soil layer. Water percolation increased rapidly when water content in the coarse layer was higher than a critical value, which could be beneficial for salt leaching but unfavorable for water retention in the root zone. These results could help understand the influence of the coarse‐textured soil layer on root zone water storage and salt leaching and provide guidance on the irrigation scheduling for salt leaching and crop growth in areas with salinization problems and shallow groundwater tables.

Climate-controlled root zone parameters show potential to improve water flux simulations by land surface models

Abstract. The root zone storage capacity (Sr) is the maximum volume of water in the subsurface that can potentially be accessed by vegetation for transpiration. It influences the seasonality of transpiration as well as fast and slow runoff processes. Many studies have shown that Sr is heterogeneous as controlled by local climate conditions, which affect vegetation strategies in sizing their root system able to support plant growth and to prevent water shortages. Root zone parameterization in most land surface models does not account for this climate control on root development and is based on lookup tables that prescribe the same root zone parameters worldwide for each vegetation class. These lookup tables are obtained from measurements of rooting structure that are scarce and hardly representative of the ecosystem scale. The objective of this research is to quantify and evaluate the effects of a climate-controlled representation of Sr on the water fluxes modeled by the Hydrology Tiled ECMWF Scheme for Surface Exchanges over Land (HTESSEL) land surface model. Climate-controlled Sr is estimated here with the “memory method” (MM) in which Sr is derived from the vegetation's memory of past root zone water storage deficits. Sr,MM is estimated for 15 river catchments over Australia across three contrasting climate regions: tropical, temperate and Mediterranean. Suitable representations of Sr,MM are implemented in an improved version of HTESSEL (Moisture Depth – MD) by accordingly modifying the soil depths to obtain a model Sr,MD that matches Sr,MM in the 15 catchments. In the control version of HTESSEL (CTR), Sr,CTR is larger than Sr,MM in 14 out of 15 catchments. Furthermore, the variability among the individual catchments of Sr,MM (117–722 mm) is considerably larger than of Sr,CTR (491–725 mm). The climate-controlled representation of Sr in the MD version results in a significant and consistent improvement of the modeled monthly seasonal climatology (1975–2010) and interannual anomalies of river discharge compared with observations. However, the effects on biases in long-term annual mean river discharge are small and mixed. The modeled monthly seasonal climatology of the catchment discharge improved in MD compared to CTR: the correlation with observations increased significantly from 0.84 to 0.90 in tropical catchments, from 0.74 to 0.86 in temperate catchments and from 0.86 to 0.96 in Mediterranean catchments. Correspondingly, the correlations of the interannual discharge anomalies improve significantly in MD from 0.74 to 0.78 in tropical catchments, from 0.80 to 0.85 in temperate catchments and from 0.71 to 0.79 in Mediterranean catchments. The results indicate that the use of climate-controlled Sr,MM can significantly improve the timing of modeled discharge and, by extension, also evaporation fluxes in land surface models. On the other hand, the method has not been shown to significantly reduce long-term climatological model biases over the catchments considered for this study.

Technical note: Accounting for snow in the estimation of root zone water storage capacity from precipitation and evapotranspiration fluxes

Abstract. A common parameter in hydrological modeling frameworks is root zone water storage capacity (SR[L]), which mediates plant water availability during dry periods as well as the partitioning of rainfall between runoff and evapotranspiration. Recently, a simple flux-tracking-based approach was introduced to estimate the value of SR (Wang-Erlandsson et al., 2016). Here, we build upon this original method, which we argue may overestimate SR in snow-dominated catchments due to snow melt and evaporation processes. We propose a simple extension to the method presented by Wang-Erlandsson et al. (2016) and show that the approach provides a lower estimate of SR in snow-dominated watersheds. This SR dataset is available at a 1 km resolution for the continental USA, along with the full analysis code, on the Google Colab and Earth Engine platforms. We highlight differences between the original and new methods across the rain–snow transition in the Southern Sierra Nevada, California, USA. As climate warms and precipitation increasingly arrives as rain instead of snow, the subsurface may be an increasingly important reservoir for storing plant-available water between wet and dry seasons; therefore, improved estimates of SR will better clarify the future role of the subsurface as a storage reservoir that can sustain forests during seasonal dry periods and episodic drought.

Plants as sensors: vegetation response to rainfall predicts root-zone water storage capacity in Mediterranean-type climates

In Mediterranean-type climates, asynchronicity between energy and water availability means that ecosystems rely heavily on the water-storing capacity of the subsurface to sustain plant water use over the summer dry season. The root-zone water storage capacity ( [L]) defines the maximum volume of water that can be stored in plant accessible locations in the subsurface, but is poorly characterized and difficult to measure at large scales. Here, we develop an ecohydrological modeling framework to describe how mediates root zone water storage (S [L]), and thus dry season plant water use. The model reveals that where is high relative to mean annual rainfall, S is not fully replenished in all years, and root-zone water storage and therefore plant water use are sensitive to annual rainfall. Conversely, where is low, S is replenished in most years but can be depleted rapidly between storm events, increasing plant sensitivity to rainfall patterns at the end of the wet season. In contrast to both the high and low cases, landscapes with intermediate values are predicted to minimize variability in dry season evapotranspiration. These diverse plant behaviors enable a mapping between time variations in precipitation, evapotranspiration and , which makes it possible to estimate using remotely sensed vegetation data − that is, using plants as sensors. We test the model using observations of in soils and weathered bedrock at two sites in the Northern California Coast Ranges. Accurate model performance at these sites, which exhibit strongly contrasting weathering profiles, demonstrates the method is robust across diverse plant communities, and modes of storage and runoff generation.

Operational soil moisture modeling using a multi-stage approach based on the generalized complementary principle

A higher drought risk in Java Island is generally known than the other regions in Indonesia. Tracking soil moisture can be an alternative way to monitor drought rather than precipitation-based drought indices. The objective of this study was to assess root-zone water storage (defined by root-zone soil moisture contents) based on a linked approach between the generalized complementary relationship (GCR) and a single bucket model in Java Island. Since it does not require precipitation for estimating actual evapotranspiration (ETa), the GCR allowed implementation of a simple single bucket model. The ETa and root-zone soil moisture estimated in this study were compared against the Global Land Evaporation Amsterdam Model (GLEAM) and the root-zone water storage additionally compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 reanalysis data products. Overall, the GCR ETa estimates were higher than those from GLEAM, and similar patterns of the root-zone water storage were found in the comparisons of both GLEAM and ERA5. The comparative evaluation suggests a further study on the adjustment of Priestley-Taylor coefficient value in Java for better application of the GCR. The soil moisture estimated by the single bucket model and the root-zone soil moisture products of GLEAM were highly correlated (0.8 or greater Pearson correlation coefficients). Low root-zone water storage and high ETa rates were found in eastern Java relative to the other areas, indicating high water shortage risks in dry season. This study found that El Niño clearly contributed to the variability of the root-zone water storage in Java especially in wet seasons (December to February). It is also suggested that the proposed approach can be useful to operationally provide soil water availability in Java from readily available meteorological observations.

WAYS v1: a hydrological model for root zone water storage simulation on a global scale

Abstract. The soil water stored in the root zone is a critical variable for many applications, as it plays a key role in several hydrological and atmospheric processes. Many studies have been conducted to obtain reliable information on soil water in the root zone layer. However, most of them are mainly focused on the soil moisture within a certain depth rather than the water stored in the entire rooting system. In this work, a hydrological model named the Water And ecosYstem Simulator (WAYS) is developed to simulate the root zone water storage (RZWS) on a global scale. The model is based on a well-validated lumped model and has now been extended to a distribution model. To reflect the natural spatial heterogeneity of the plant rooting system across the world, a key variable that influences RZWS, i.e., root zone storage capacity (RZSC), is integrated into the model. The newly developed model is first evaluated based on runoff and RZWS simulations across 10 major basins. The results show the ability of the model to mimic RZWS dynamics in most of the regions through comparison with proxy data, the normalized difference infrared index (NDII). The model is further evaluated against station observations, including flux tower and gauge data. Despite regional differences, generally good performance is found for both the evaporation and discharge simulations. Compared to existing hydrological models, WAYS's ability to resolve the field-scale spatial heterogeneity of RZSC and simulate RZWS may offer benefits for many applications, e.g., agriculture and land–vegetation–climate interaction investigations. However, the results from this study suggest an additional evaluation of RZWS is required for the regions where the NDII might not be the correct proxy.

Development and application of the soil moisture routing (SMR) model to identify subfield-scale hydrologic classes in dryland cropping systems using the Budyko framework

Crop yield patterns at the field level in dryland agricultural systems often trace back to the spatial structure of water storage and subsurface flow paths. These lateral flow paths create zones of saturation that promote nutrient loss from runoff and leaching beneath the root zone. Despite its clear importance, a spatially explicit representation of landscape hydrology is frequently missing from site-specific crop and fertilizer management plans. This work uses the grid-based water balance model, SMR (soil moisture routing), to adapt an established method of hydrologic classification, the Budyko framework, for small agricultural watersheds. The model was evaluated with spatially distributed soil moisture data and surface runoff measurements in four annually cropped fields located across the eastern Palouse region of the inland Pacific Northwest. The Budyko framework, modified to include change in root zone water storage and lateral flow, was combined with a separate indicator of yield potential to form a dual-threshold classification scheme for variable-rate fertilizer management that balances agronomic benefit with environmental impact. Each field was divided into zones based on crop yield and nitrogen loss potentials, which link to the hydrologic functioning of the landscape through soil water storage and subsurface redistribution, respectively. Results showed large variability in the annual water balance assessed within individual fields and between fields. The greatest variability was observed in the two eastern-most fields where the wettest zone of each had water supply 172% and 137% that of the dry zone. Depth to a hydraulic restrictive layer and topography were identified as primary landscape controls on the water supply (precipitation plus net lateral flow minus change in water storage) in these fields. Moreover, the steep climate gradient in the region created field-to-field variability in water balance with the ratio of evapotranspiration to precipitation ranging from 91% in the western-most field to approximately 70% in each of the fields to the east. The clearest choice is for growers to apply more fertilizer in areas with high water storage capacity and high crop yield and where fertilizer loss through leaching and runoff is minimal. Less fertilizer should be applied in areas with low water storage capacity that frequently saturate. Areas with high storage capacity and frequent saturation involve a tradeoff between yields and minimizing fertilizer loss that may be best served by splitting fertilizer application to avoid excessive over-winter leaching losses. Even though remote sensing and variable-rate applicators are at the frontiers of precision agriculture, landscape hydrology underlies all dryland crop production and should accordingly be looked to for inspiration in the design of cropping systems.

Effects of Rainfall-Induced Topsoil Structure Changes on Root-Zone Moisture Regime during the Dry Period

Rainfall erosion and subsequent intermittent drought are serious barriers for agricultural production in the subtropical red soil region of China. Although it is widely recognized that rainfall-induced soil structure degradation reduced soil water storage and water-holding capacity, the effects of variation of the rainfall-induced topsoil structure on the subsequent soil water regime during the dry period is still rarely considered. The objective of this study was to ascertain the way of rainfall-induced topsoil structure changes on the subsequent soil water regime during the dry period. In a three-year-long experiment, six practices (CK, only crop; SM, straw mulching; PAM, polyacrylamide surface application; B, contour Bahia-grass strip; SPAM, straw mulching and polyacrylamide surface application; and BPAM, contour Bahia-grass strip and polyacrylamide surface application) were conducted at an 8° farmland with planting summer maize resulting in different topsoil structure and root-zone moisture, to establish and reveal the quantitatively relationship between the factors of topsoil structure and soil drought. Rainfall erosion significantly increased the soil crust coverage, and decreased the WSA0.25, 0–30 mm soil porosity and mean pore size. There was no significant difference during the raining stage of root-zone water storage between CK and other practices. An index of soil drought intensity (I) and degree (D) was established using soil water loss rate and soil drought severity. The larger value of I means a higher rate of water loss. The larger value of D means more severe drought. During the dry period, I and D were significantly higher in CK than in other practices. I and D had significantly positively correlation with the crust size and crust coverage, and negatively with WSA0.25, 15–30 mm soil porosity and mean pore size. Among of soil structure factors, the soil porosity had the largest effect on I and D. The rainfall-induced topsoil structure changes greatly deteriorated the root-zone regime during the dry period mainly due to significant increasing soil water loss but little improving the raining stage of soil water storage. Straw mulching had greater effects than other practices in alleviating rainfall-induced erosion and intermittent drought, and could be a better strategy applied for this region.

New interpretation of the role of water balance in an extended Budyko hypothesis in arid regions

Abstract. The Budyko hypothesis (BH) is an effective approach to investigating long-term water balance at large basin scale under steady state. The assumption of steady state prevents applications of the BH to basins, which is unclosed, or with significant variations in root zone water storage, i.e., under unsteady state, such as in extremely arid regions. In this study, we choose the Heihe River basin (HRB) in China, an extremely arid inland basin, as the study area. We firstly use a calibrated and then validated monthly water balance model, i.e., the abcd model, to quantitatively determine annual and monthly variations of water balance for the sub-basins and the whole catchment of the HRB, and find that the roles of root zone water storage change and that of inflow from upper sub-basins in monthly water balance are significant. With the recognition of the inflow water from other regions and the root zone water storage change as additional possible water sources to evapotranspiration in unclosed basins, we further define the equivalent precipitation (Pe) to include local precipitation, inflow water and root zone water storage change as the water supply in the Budyko framework. With the newly defined water supply, the Budyko curve can successfully describe the relationship between the evapotranspiration ratio and the aridity index at both annual and monthly timescales, whilst it fails when only the local precipitation being considered. Adding to that, we develop a new Fu-type Budyko equation with two non-dimensional parameters (ω and λ) based on the deviation of Fu's equation. Over the annual timescale, the new Fu-type Budyko equation developed here has more or less identical performance to Fu's original equation for the sub-basins and the whole catchment. However, over the monthly timescale, due to large seasonality of root zone water storage and inflow water, the new Fu-type Budyko equation generally performs better than Fu's original equation. The new Fu-type Budyko equation (ω and λ) developed here enables one to apply the BH to interpret regional water balance over extremely dry environments under unsteady state (e.g., unclosed basins or sub-annual timescales).

Interacting Vegetative and Thermal Contributions to Water Movement in Desert Soil

Vadose Zone J. 10:552–564 (2011) The term non-vegetated in the figure legends of the original version (Fig. 4 and Fig. 9b and first-page image) has been replaced by unvegetated to be consistent with the text. The complete corrected figures are: Simulated root-zone (0–1-m depth) water storage for vegetated and unvegetated soil and measured daily precipitation (Johnson et al., 2007). Simulated cumulative total subsurface water flux during a 4.7-yr period for (A) vegetated and (B) unvegetated soil. Fluxes are for simulations with heat flow in layered and homogeneous soil and without heat flow in layered soil. Sign convention is that downward fluxes are negative.

Modeling the monthly mean soil-water balance with a statistical-dynamical ecohydrology model as coupled to a two-component canopy model

Abstract. The statistical-dynamical annual water balance model of Eagleson (1978) is a pioneering work in the analysis of climate, soil and vegetation interactions. This paper describes several enhancements and modifications to the model that improve its physical realism at the expense of its mathematical elegance and analytical tractability. In particular, the analytical solutions for the root zone fluxes are re-derived using separate potential rates of transpiration and bare-soil evaporation. Those potential rates, along with the rate of evaporation from canopy interception, are calculated using the two-component Shuttleworth-Wallace (1985) canopy model. In addition, the soil column is divided into two layers, with the upper layer representing the dynamic root zone. The resulting ability to account for changes in root-zone water storage allows for implementation at the monthly timescale. This new version of the Eagleson model is coined the Statistical-Dynamical Ecohydrology Model (SDEM). The ability of the SDEM to capture the seasonal dynamics of the local-scale soil-water balance is demonstrated for two grassland sites in the US Great Plains. Sensitivity of the results to variations in peak green leaf area index (LAI) suggests that the mean peak green LAI is determined by some minimum in root zone soil moisture during the growing season. That minimum appears to be close to the soil matric potential at which the dominant grass species begins to experience water stress and well above the wilting point, thereby suggesting an ecological optimality hypothesis in which the need to avoid water-stress-induced leaf abscission is balanced by the maximization of carbon assimilation (and associated transpiration). Finally, analysis of the sensitivity of model-determined peak green LAI to soil texture shows that the coupled model is able to reproduce the so-called "inverse texture effect", which consists of the observation that natural vegetation in dry climates tends to be most productive in sandier soils despite their lower water holding capacity. Although the determination of LAI based on complete or near-complete utilization of soil moisture is not a new approach in ecohydrology, this paper demonstrates its use for the first time with a new monthly statistical-dynamical model of the water balance. Accordingly, the SDEM provides a new framework for studying the controls of soil texture and climate on vegetation density and evapotranspiration.

Development of models to predict Pinus radiata productivity throughout New Zealand

Development of spatial surfaces describing variation in productivity across broad landscapes at a fine resolution would be of considerable use to forest managers as decision support tools to optimize productivity. In New Zealand, the two most widely used indices to quantify productivity of Pinus radiata D. Don are Site Index and 300 Index. Using an extensive national data set comprising a comprehensive set of national extent maps, multiple regression models and spatial surfaces of these indices for P. radiata were constructed. The final models accounted for 64% and 53%, respectively, of the variance in Site Index and 300 Index. For Site Index, variables included in the final model in order of importance were mean annual air temperature, fractional mean annual available root-zone water storage, mean annual windspeed, length and slope factor, categories describing Land Environments of New Zealand (LENZ), and major soil parent material. The variables included in the final model of 300 Index in order of importance included the degree of ground frost during autumn, fractional mean annual available root-zone water storage, categories describing LENZ, vegetation classification, foliar nitrogen, taxonomic soil order, and major soil parent material. These results highlight the utility of thematic spatial layers as driving variables in the development of productivity models.

Comparisons of two soil water flow models under variable irrigation

Performance of WATCOM (a numerical model) and CRPSM (a simple water balance model) were assessed in simulating root zone water storage and water balance components under cowpea in Nigeria using a line source sprinkler system. Three sets of field data were collected: the first was used for calibration and model parameters’ estimation and the other two for testing and comparisons. The simulated soil water storage and crop evapotranspiration with WATCOM and CRPSM were in good agreement with field-measured data though WATCOM performed significantly better (P < 0.05) under the stressed condition. The maximum average error between predicted and measured soil water storage was −0.95 and +1.47 mm for WATCOM and CRPSM, respectively, while that between measured and predicted actual crop evapotranspiration was +2.7 and +11.38 mm, respectively, for the two models. WATCOM gave generally higher cumulative deep percolation and lower evapotranspiration than that of CRPSM for all irrigation levels (P < 0.05), and values of deep percolation for WATCOM were in better agreement with field data than that of CRPSM. This suggests that drainage below the field capacity needs to be included in CRPSM and that WATCOM will be a more useful management tool when detailed soil parameter is required and under variable water regime.