H33C-1445
Identification of Diurnal, Seasonal and Inter-Annual Variability Across SE Asian Field Observations of key Water Cycle Variables: Rainfall, net Radiation, Total Evaporation and River Discharge
The identification of periodic patterns in water cycle variables is critical to the understanding of land-atmosphere interactions, climate change and the evaluation of General Circulation Model (GCM) output. SE Asia in particular plays a very important role on the global climate because it is a large source of energy and water fluxes into the upper atmosphere. Cycle identification is carried out following the Data Based Mechanistic (DBM) philosophy, which focuses on the use of parsimonious, rigorous models which are characterised by lack of a priori assumptions, built in uncertainty analysis and final model acceptance dependent on the physical interpretation of the results. The DBM tool used here is the Unobserved Component - Dynamic Harmonic Regression (UC-DHR) model, which is a statistical method that allows the identification of variability in time series by introducing Time Variable Parameter (TVP) estimation of harmonic components. UC-DHR is not scale dependent and was thus applied to both hourly (to investigate diurnal variation) and fortnightly datasets (for intra- and inter-annual variability). The data used in the analysis has been gathered from existing catchment datasets for three regions of tropical SE Asia, namely Northern Thailand, Central Peninsular Malaysia and Northeast Borneo. These regions were chosen because they represent the hydro-climatic gradient (seasonal to equatorial) present within the tropics and because SE Asia has the most extensive set of catchment/plot studies within the humid tropics. Results show modeling tools were able to quantify the main patterns present in the observations throughout different time scales (diurnal, intra-annual and inter-annual) and the strength of the correlation pattern between the four hydro-climatic variables. The subsequent discussion focuses on the physical processes behind those patterns (e.g. diurnal variability caused by local convection due to solar heating; impact of El Niño Southern Oscillation on inter-annual variability of rainfall and river discharge).
H33C-1446
Long-Term Estimation of Soil Heat Flux Using Single Layer Time Series Data of Soil Temperature
Soil heat flux may play an important role in surface energy balance. In this study, we examined the performances of two methods for predicting soil heat flux from single layer time series data of soil temperature. The first one is the traditional method, which an analytical solution of soil heat flux can be obtained by assuming the surface soil temperature varies sinusoidally. The second one is the connection between surface soil temperature and soil heat flux derived by half-order time derivative/integral, and is based on a simple model of heat transfer described by a one-dimensional diffusion equation with a constant heat diffusivity. Good agreements between measured and predicted soil heat fluxes were found for both methods. However, it was shown that the half-order derivative method has a better capability to capture flux accuracy and trend than the traditional method for long-term soil heat flux estimation. Keywords: soil heat flux, half-order derivative method, soil temperature
H33C-1447
Seasonality and Remoteness/Locality of Land-Atmosphere Coupling Strength
In this study, we estimate the seasonal variation of land-atmosphere coupling strength using an extended series of atmospheric general circulation model (AGCM) simulations. In the western Sahel of Africa, strong coupling strength for precipitation is found in April and May, just prior to and at the beginning of the monsoon season. At this time, unstable conditions in the lower level of troposphere, as induced by local land state, allow heat and water fluxes from the land surface to influence the variability of convective precipitation - and thus the timing of monsoon onset. We also investigate the potentially non-local nature of land-atmosphere coupling strength. Our AGCM experiments show that the prescription of land surface states in the Tibetan Plateau in June has a strong remote impact on the geopotential height variability at 500hPa height over eastern China and Japan, where strong westerly wind fields are formed by subtropical and polar jets. A remote impact is also seen in the 850 hPa height fields there. The geographical distribution of Tibetan Plateau impact corresponds to the Meiyu/Baiu fronts in June, which suggests that the prescription of the Tibetan Plateau's land surface states may help guide East Asian summer monsoon activity.
H33C-1448
Rainfall Interception by Corn Plants. First Part : Development of a Method to Quantify the Stemflow in Corn.
When rain falls in a crop, it is redistributed into three elements, namely stemflow, throughfall, and plant storage. While the throughfall reaches the soil in a similar way to unmodified rainfall, the stemflow concentrate the water towards the root zone and the water stored in the plant surface evaporates. This suggests that the interaction of rainfall with vegetation influences processes of great concern to environmental scientists and engineers such as evapotranspiration, rainfall-runoff relationships, distribution of soil moisture, soil erosion, ground water quality, and others. Our work was done in two stages. The first was purely qualitative, when the interaction of rainfall with the crop was observed for different stages of plant development. The second stage involved the development of an instrument to measure the stemflow rate and accumulation. The design of the stemflow meter was based on a tipping bucket rain gauge modified to collect the water flowing on the stem of the plant instead of rain. We installed the prototype in a fully developed plant and a regular rain gauge in the vicinities of the field to be used as reference. There is no consensus in the current literature on how to compare the volumetric stemflow rate to rain gauge data. We show some methods and propose one that we consider suitable for fully developed plants. So far our results suggest that the stemflow accounted for almost 90% of the total rainfall, being an important element of the partitioning of rainfall by the considered plant. It is to be noted that our experiment is in early stages and our results reflect the behavior of one single plant and cannot be generalized to the whole field and to other stages of development. A complete experiment with independent measurements of stemflow, throughfall, rainfall and other variables is yet to come.
H33C-1449
SABAE-HW3D: a Meteor-Hydrological Model Coupling the Land Surface to Groundwater Flow
A new coupled model linking the land surface scheme SABAE-HW with the saturated groundwater flow is introduced. SABAE-HW stands for: Soil Atmosphere Boundary, Accurate Evaluations of Heat and Water. It was presented (Loukili et al., 2007) as a parallel extension to the Canadian LAnd Surface Scheme (CLASS) which permits a free choice of soil column depth and layers. This development is important as it allows for the coupling of the atmosphere, land surface and subsurface zones. The generic domain under study is subdivided into soil columns whose surface areas represent GCM grid squares. The depths of columns extend to the saturated zone where a water table lower boundary condition is prescribed. In each column the unsaturated flow is forced by the corresponding meteorological data, and the moisture (liquid and frozen) and temperature profiles are computed by SABAE-HW using the half hour time step. The horizontal flow in the saturated zone is described by the vertically integrated model incorporating a storage term, and discretized using a finite volume scheme operating on the same GCM quadrilateral mesh. Since the saturated flow model affords larger time steps, bottom drainages from soil columns are summed up and input as cell recharges. Meanwhile, as the water table fluctuates, the individual column's mesh is allowed to deform. This physically based coupling strategy was selected for its superiority in providing stable and consistent results. In fact, the convergence to steady state situations in both the unsaturated and saturated zones is realized in many validation numerical tests. Moreover, our SABAE-HW3D code was successfully benchmarked against the finite element code Seep/W (Geo-Slope International, 2002) for steady and transient groundwater flows through different soil types. Even when handling coarse grids, SABAE-HW3D solutions are free of moisture oscillation and under or overshoot near the water table. The model is used to understand and assess properly regional moisture exchanges between the atmosphere and the ground in the Canadian Prairies. The ultimate goal is to investigate the onset and persistence of drought.
H33C-1450
Atmosphere-Ground Interaction in the Context of Convective Precipitation Using a Soil Moisture and Energy Balance Network During COPS
Temporal and spatial variability of soil moisture are important parameters within the soil-atmosphere system, influencing the hydrology on small to medium scales as well as the availability of moisture within the atmosphere. The latter may have significant influence on the local and the mesoscale, e.g. concerning the initiation of thermally induced wind regimes, and in connection with the initiation of convective systems. In spite of the importance, there is still an almost complete lack of operational soil moisture observations within the meteorological community. General lack of data is probably because spatial soil moisture variability is large, depending strongly on soil type, surface characteristics, land use and vegetation. For soil moisture monitoring purposes two approaches are common: (1) spatially integrating remote sensing methods on larger scales and (2) in-situ measurements on smaller scales (point measurements), the latter being generally more accurate, but limited to the near-field of the sensor. Hydrogeophysical methods are used on an intermediate scale, because of their applicability for 2D and 3D problems (penetration depths of several meters) and their flexible spatial scales (between the centimetre scale and a few kilometres). Within a large experimental and modelling investigation of the influence of soil moisture variability on convection initiation in orographic terrain, a soil moisture network was installed within the international Convective and Orographically-induced Precipitation Study (COPS) campaign, which was conducted in summer 2007 in south- western Germany and eastern France. The COPS experiment on warm season precipitation formation provides a unique data set documenting the lifecycle of convective systems in the atmosphere. With particular emphasis on its initiation, precipitating convection is investigated in response to forcing by mountain effects, the state of soil and vegetation. As convective precipitation formation can be initiated by (amongst others) (i) surface heating and moisture supply overcoming convective inhibition during potentially instable stratification as well as (2) hot spots at the surface and low-level flow convergence over mountains crests, energy balance and moisture availability at the surface may play a critical role in cases of strong local atmosphere-ground coupling (= weak mesoscale forcing). Selected COPS cases will be analysed to decide on the controlling factors, which have to be adequately represented in models for quantitative precipitation forecast (QPF). In this contribution we will introduce the soil moisture monitoring network, including a simplified soil moisture probe (SISOMOP) and several innovative techniques for spatial soil moisture measurements, and will present first results on atmosphere-ground interaction in orographic terrain.
H33C-1451
Is Regional Groundwater Flow a Significant Component of the Water Budget in a River Basin?
The spatial organization and temporal memory of the groundwater reservoir, and its interaction with the surface water has an integral role in the lateral transport of continental water and energy, affecting soil moisture distributions, evapotranspiration, precipitation and stream discharge across the land surface. The current climate models are unable to account for this lateral component, and route Precipitation (P) minus Evapotranspiration (ET) directly to stream discharge; hence, a separation of groundwater flow from surface water flow is necessary to asses the relative importance of each reservoir across a given continent. Here we use the ratio of P-ET (surface recharge, or R) to stream discharge (Qr) to evaluate the importance of the groundwater component within a given basin; where Qr/R = 1 we surmise that all the atmospheric surplus exits a basin through river discharge; where Qr/R < 1, a basin is considered a groundwater exporter; and where Qr/R > 1, a basin is considered a groundwater importer. In this study, 39 years of USGS HCDN annual mean observed stream discharge (naturalized) from 1555 basins across the continental U.S are removed from total surface recharge, derived from VIC simulation, yielding the portion of surface recharge leaving a basin via subsurface pathways. It was found that the Qr/R ratio deviates significantly from 1 across the contiguous 48 U.S. states. Detailed investigations of individual basins across the continent suggest that deviations of the Qr/R ratio from 1 are primarily a function of the subsurface geology, while climate and basin scale influence the magnitude of those deviations. Further, a marked incongruity between the surface drainage flow direction and groundwater flow direction is apparent where regional groundwater flow has developed, suggesting that surface drainage as a result of elevation is only partially indicative of subsurface flow regimes. This apparent significance of the groundwater component reinforces the need for inclusion of the groundwater reservoir in the current climate models.
H33C-1452
Understanding of accuracy on calculated soil moisture field for the study of land-atmosphere interaction
Understanding the state of soil moisture is effective to enhance climate predictability on inter-seasonal or annual time scales. Thus, the Global Soil Wetness Project (GSWP) has been implemented as an environmental modeling research activity. The SiBUC (Simple Biosphere including Urban Canopy) land surface model is one of the participants of the 2nd GSWP, and it uses mosaic approach to incorporate all kind of land-use. In order to estimate the global soil moisture field as accurately as possible and to utilize the products of GSWP2 simulation more efficiently, SiBUC is run with irrigation scheme activated. Integration of one-way uncoupled SiBUC model from 1986 to 1995 have produced global soil moisture field. Both the model and forcing data may contain uncertainty. However, the SiBUC model is one of the few models which can consider irrigation effect. And also, the advantage of the meteorological forcing data provided from GSWP2 is hybridization among reanalysis products, observation data and satellite data. In this sense, it is assumed that GSWP2 products is the most accurate global land surface hydrological data set in available. Thus, these global products should be applied to land-atmosphere interaction study, if possible. To do this, it is important to understand inter-annual or much higher time scale accuracy on calculated soil moisture filed. In this study, calculated soil moisture field are validated with observation of soil moisture in five regions (Illinois:USA, China, India, Mongolia, Russia). The Russian data has two types data: one is located in spring wheat and another is located in winter wheat. These observation data are provided from Global Soil Moisture Data Bank (GSMDB). To understand the time scale accuracy on soil moisture field, three correlation coefficients are calculated between calculated soil moisture and observed soil moisture: inter-annual, inter-seasonal and monthly mean correlation, respectively. As a result, if the median value in each region is focused on, high monthly correlation are shown in Illinois (0.83) and India (0.75). In these regions, inter-seasonal correlation is also high, but inter-annual correlation becomes lower. On the other hand, in China or Mongolia, all median value of correlation is low. And, both types of monthly correlation in Russia are relatively high (0.69, 0.65). In addition, inter-seasonal and inter-annual correlation are almost same as monthly correlation. From the result, from the viewpoint of regional scale, calculated soil moisture field in Illinois and India have high accuracy on monthly and inter-seasonal time scales. In Russia, calculated soil moisture field has relatively high accuracy on any time scales. Low accuracy on monthly time scale in China and Mongolia correspond to the result of multi-model analysis and validation (Guo et al., 2007). From this concurrent result, it is assumed that it is difficult to estimate the soil moisture field in China and Mongolia for any models or meteorological forcing data have some uncertainty. Nevertheless these reason should be investigated.
H33C-1453
An Examination of Water Table Hydrographs and Water use by Phreatophytes
Connections between water use by phreatophyte communities and the groundwater they draw upon are receiving attention from ecohydrologists and resource managers. Since 2000, we have measured evapotranspiration (ET) from eddy covariance flux towers to estimate water use and produced numerous water table (WT) hydrographs from monitoring wells at several sites dominated by pheatophytes along the Rio Grande floodplain of central New Mexico, USA. The four sites presented here support large areas of vegetation and include riparian reaches dominated by native cottonwood or by introduced, invasive species (saltcedar, Russian olive). Several growing seasons have coincided with low runoff due to moderate to extreme drought, with one abnormally wet season resulting in extensive flooding at two sites. On an annual (full growing season) basis, total ET generally declines with mean WT depth within each site, with strong correlations at two sites. Across all sites, WT depth is not a good indicator of ET, which is similar at the shallowest (1.2 m) and deepest (2.4 m) sites. These are the two strongly correlated sites. Both have by far the greatest magnitude WT elevation range, and are inundated by flooding during wet years. The seasonal range as a ratio to mean WT depth at these sites are 1.3 to 1.5, compared to 0.3 for the other two sites. Spring runoff flooding resulted in a significant increase in ET at a young, emerging Russian olive site, but had no effect on ET at a mature, monospecific saltcedar site. Finally, at the two sites with little seasonal range in WT elevations, we investigated ET and WT relationships on a daily basis. Preliminary results estimating ET from diurnal WT fluctuations compared with ET measured by the flux tower are presented. Overall, our results indicate that the relationship between annual ET and WT depths is more robust in locations with dynamic WT elevations and vadose zones. Also, flooding does not necessarily increase ET in some phreatophyte communities.
H33C-1454
Using Baseflow to Constrain Water Table Depth Simulations in the NCAR Community Land Model (CLM)
Several recent studies have shown the importance of representing groundwater in land surface hydrologic simulations. However, optimal methods for model calibration in order to realistically simulate baseflow and groundwater depth have received little attention. Moreover, due to model parameter interactions, various parameter combinations are found to exhibit equifinality in simulated total runoff. In this study a simple lumped groundwater model was incorporated into the Community Land Model (CLM), in which the water table is interactively coupled to soil moisture through the groundwater recharge fluxes. The coupled model (CLMGW) is successfully validated in Illinois against a 22-year (1984~2005) monthly observational dataset. The advantage obtained from incorporating baseflow calibration in addition to traditional calibration based on measured streamflow alone is demonstrated. Using the optimal Pareto parameter sets identified from baseflow and total flow calibration, the flow partitioning and water table depth simulation by the CLMGW are improved, and the equifinality problem is alleviated. For other regions that lack observations of water table depth, the baseflow estimates can be used to enhance parameter estimation. The calibrated CLMGW is applied to the entire US to study the impact of groundwater on land hydrologic memory.
H33C-1455
Selecting The Best Meteorological Forcing Dataset For Each River Basin
One of the most important goal of Global Soil Wetness Project (GSWP) is to produce state-of-the-art global data sets of land surface fluxes, state variables and related hydrological quantities for 10-year period (1986-1995). Due to the time schedule and huge amount of data sets, GSWP simulation had started without enough validation and quality check of the provided forcing data. Considering the goal of GSWP, baseline simulation should be run by the "best" dataset. Off course, there is no "perfect" global dataset. Information on the bias and accuracy in the forcing dataset will be helpful for the data analysis. In the previous study, forcing data sets of GSWP-2 are analyzed and compared with surface measurement in terms of monthly mean value and day-to-day variation within one month. And the rank of the accuracy of each dataset was shown for each continent. By the way, the accuracy of the data varies from place to place within the same continent. In this study, same kind of analyses were applied for each river basin. And the best combination of dataset will be produced for each river basin. Also, we try to remove the biases detected in the data analyses for enabling the quantitative use of hydrological application. http://grads.iges.org/gswp2/
H33C-1456
Reference Canopy Stomatal Conductance Explains Spatiotemporal Patterns of Tree Transpiration
Increased heterogeneity in patterns of whole tree transpiration (EC) with increasing atmospheric vapor pressure deficit (D) suggests a dynamic response of sap flow velocity (JS) to environmental drivers. We hypothesized that differences in reference stomatal conductance (GSref), stomatal conductance at D = 1kPa, would explain the spatiotemporal dynamics of JS. Using a coupled model of plant hydraulic and biochemical processes we tested this hypothesis with sap flux data for 106 aspen ( Populus tremuloides) and 108 sugar maple ( Acer saccharum) trees collected from plots using in 2-D cyclic sampling scheme during the summer of 2005 in northern Wisconsin. Inverse modeling is used to estimate GSref for each tree. For each species, trees from across the ranges of JS and diameter distributions are compared. GSref explained temporal variability in spatial patterns of EC We explore several possible mechanistic explanations for differences in GSref among trees. Topoedaphic factors are considered to determine if location within a stand has an effect. We also consider competition with neighboring individuals as a possible explanation. Variations in GSref in aspen were explained in part by competition for light between neighboring individuals, while competition for light was not a significant factor for sugar maple. Based on simulation analysis we identify possible biochemical feedbacks as drivers of the variability in plant hydraulics. Other factors examined included micro-topography within both sites.
H33C-1457
Radiative-convective-land surface equilibria: An idealized modeling approach
The concept of radiative-convective equilibrium has been a cornerstone in our understanding of the climate of the Tropical oceans for decades. Our hypothesis is that there are analagous equilibrium states over semi-arid continental regions during summer that link groundwater, plant ecosystems, geomorphology, and the atmosphere. We test this hypothesis using an idealized version of the Weather Research and Forecast model (WRF) coupled to the NOAH Land Surface Model. A control simulation of radiative-convective equilibrium over the ocean yields results that are similar to other studies and is used for comparison to two idealized land surface configurations. Both configurations represent a mixed shrubland-grassland environment, but the first is characterized by a relatively high vegetation fraction and a sandy clay loam soil, while the second configuration consists of a much lower vegetation fraction and sandy soil. In this presentation, we will describe the nature of the equilibrium states attained in the model in terms of daily rain rates, surface energy fluxes, and groundwater storage and we will show the impact of changing land surface conditions on those equilibrium states. This modeling approach may thus be a useful framework for evaluating feedbacks between ecosystems and climate change in semi-arid regions.
H33C-1458
Simulations of the Amazon Hydrologic Cycle Using a Coupled Land-Atmosphere General Circulation Model
Land-atmosphere interactions have a profound effect on our weather and climate. Previous modeling studies have shown that these interactions are of particular importance in the Amazon Basin, where the predicted effects of climate change include reduced precipitation. Specifically, moisture fluxes from the land surface both affect and are affected by the large-scale circulation and precipitation. The rainforest is already fairly drought resistant, being able to withstand several months in a row with little to no rainfall. Understanding the mechanisms that allow this resistance is important for planning for the future climate. To study these interactions, we have coupled a land surface model (the Simple Biosphere Model – SiB3), to Colorado State University's General Circulation Model (BUGS5). In a previously coupled version of the models, rainfall gradually decreased in the Amazon Basin over a three-year period due to decreased moisture recycling, until the wet season was indistinguishable from the dry season. SiB3 includes improved representations of the vertical distribution of groundwater, more realistic root profiling and an improved response of the vegetation to drought stress. Results from multiyear runs show that these improvements allow the forest to function more realistically and that the representation of the Amazon hydrologic cycle is improved. Attention is paid to particular changes within the land surface model and their effects on the results.