The Plumbing of Land Surface Models Best, M. J.; Abramowitz, G.; Johnson, H. R. ...
Journal of hydrometeorology,
06/2015, Letnik:
16, Številka:
3
Journal Article
Recenzirano
Odprti dostop
The Protocol for the Analysis of Land Surface Models (PALS) Land Surface Model Benchmarking Evaluation Project (PLUMBER) was designed to be a land surface model (LSM) benchmarking intercomparison. ...Unlike the traditional methods of LSM evaluation or comparison, benchmarking uses a fundamentally different approach in that it sets expectations of performance in a range of metrics a priori—before model simulations are performed. This can lead to very different conclusions about LSM performance. For this study, both simple physically basedmodels and empirical relationships were used as the benchmarks. Simulations were performed with 13 LSMs using atmospheric forcing for 20 sites, and then model performance relative to these benchmarks was examined. Results show that even for commonly used statistical metrics, the LSMs’ performance varies considerably when compared to the different benchmarks. All models outperform the simple physically based benchmarks, but for sensible heat flux the LSMs are themselves outperformed by an out-of-sample linear regression against downward shortwave radiation. While moisture information is clearly central to latent heat flux prediction, the LSMs are still outperformed by a three-variable nonlinear regression that uses instantaneous atmospheric humidity and temperature in addition to downward shortwave radiation. These results highlight the limitations of the prevailing paradigm of LSM evaluation that simply compares an LSM to observations and to other LSMs without a mechanism to objectively quantify the expectations of performance. The authors conclude that their results challenge the conceptual view of energy partitioning at the land surface.
Observing system simulation experiments were used to investigate ensemble Bayesian state-updating data assimilation of observations of leaf area index (LAI) and soil moisture (θ) for the purpose of ...improving single-season wheat yield estimates with the Decision Support System for Agrotechnology Transfer (DSSAT) CropSim-Ceres model. Assimilation was conducted in an energy-limited environment and a water-limited environment. Modeling uncertainty was prescribed to weather inputs, soil parameters and initial conditions, and cultivar parameters and through perturbations to model state transition equations. The ensemble Kalman filter and the sequential importance resampling filter were tested for the ability to attenuate effects of these types of uncertainty on yield estimates. LAI andθobservations were synthesized according to characteristics of existing remote sensing data, and effects of observation error were tested. Results indicate that the potential for assimilation to improve end-of-season yield estimates is low. Limitations are due to a lack of root zone soil moisture information, error in LAI observations, and a lack of correlation between leaf and grain growth.
Information about the ratio of transpiration (
T) to total evapotranspiration (
T/ET) is related to critical global change concerns, including shrub encroachment and non-native species invasion. In ...this study, a new approach was developed to partition measurements of ET into daily evaporation (
E
D) and daily transpiration (
T
D) in a semiarid watershed based on the low-cost addition of an infrared thermometer and soil moisture sensors to existing eddy covariance and Bowen ratio systems. The difference between the mid-afternoon and pre-dawn soil surface temperature (Δ
t) was used to identify days when
E
D approached a seasonal minimum (
E
Dmin) and thus,
T
D
≈
ET
D
−
E
Dmin. For other days, an empirical approach was used to partition ET
D into
E
D and
T
D based on volumetric soil moisture. The method was tested using Bowen ratio estimates of ET and continuous measurements of surface temperature with an infrared thermometer (IRT) at a grassland and shrubland site within the Walnut Gulch Experimental Watershed in southeast Arizona USA in years 2004–2006. Validation was based on a second dataset of Bowen ratio, IRT and shrub sap-flow measurements in 2003. Results showed that reasonable estimates of
T
D were obtained for a multi-year period with ease of operation and minimal cost. Estimates of
T
D and
E
D were summed over the study period when plants were actively transpiring for years 2004, 2005 and 2006 to estimate totals over the study period,
T
S and
E
S, respectively. Preliminary analysis suggests that the accuracy of
T
S estimates was 7% of the total measured sum and the precision of
T
S estimates was about 4%. For this study period,
T
S was related strongly to ET
S, with a slope of 0.79 for the grass-dominated site and 0.64 for the shrub-dominated site for the 3 years. Thus, for these sites during the study period in these years, the
T
S/ET
S was higher for the grass-dominated site than for the shrub-dominated site, and did not vary systematically with variation in amounts and timing of precipitation. The Δ
t-based partitioning method has potential for international application in other well-instrumented ecosystems but will need to be tested for application when evaporation is limited by energy rather than water.
Simulation modelers increasingly require greater flexibility for model implementation on diverse operating systems, and they demand high computational speed for efficient iterative simulations. ...Additionally, model users may differ in preference for proprietary versus open-source software environments. These issues necessitate the development of strategies to maximize model compatibility across operating systems, to ensure numerically accurate simulations for alternative compiler selections, and to understand how these choices affect computational speed. We developed an approach to evaluate model performance using diverse Fortran compilers on multiple computer operating systems. A single desktop computer with five identical hard drives was designed to permit meaningful comparisons between five operating systems while minimizing differences in hardware configuration. Three Fortran compilers and relevant software development tools were installed on each operating system. Both proprietary and open-source versions of compilers and operating systems were used. Compatibility and performance issues among compiler and operating system combinations were assessed for an example case: the Cropping System Model (CSM) as implemented in version 4.5 of the Decision Support System for Agrotechnology Transfer (DSSAT). A simulation study that included 773 simulations and assessed the full suite of crop growth modules within DSSAT-CSM was conducted for each compiler and operating system configuration. For a given simulation, results were identical for anthesis date (ADAT), maturity date (MDAT), and maximum leaf area index (LAIX) regardless of the compiler or operating system used. Over 94% of the simulations were identical for canopy weight at maturity (CWAM) and cumulative evapotranspiration at maturity (ETCM). Differences in CWAM were predominantly less than 2kgha−1 and were likely the result of differences in floating point handling among compilers. Larger CWAM discrepancies highlighted areas for improvement of the model code. Model implementations with the Intel Fortran compiler on the Linux Ubuntu operating system provided the fastest simulations, which averaged 9.0simulationss−1. Evaluating simulation models for alternative compiler and operating system configurations is invaluable for understanding model performance constraints and for improving model robustness, portability, usefulness, and flexibility.
Observations of the temporal and spatial distribution of poststorm soil moisture in open shrublands and savannas are limited, yet they are critical to understanding the interaction and feedback ...between moisture distribution and canopies. The objective of this analysis was to study the hydrologic impacts of precipitation pulses on the upper layer of soils under and between shrubs. The study was based on measurements of precipitation, runoff, and under‐ and between‐shrub soil moisture over a period of 20 years (1990–2009) at a shrub‐dominated site in the Walnut Gulch Experimental Watershed (WGEW) near Tombstone, Arizona. Within much of the root zone (to 30 cm depth), infiltration was not significantly different under versus between shrubs, and the under:between infiltration ratio was not related to pulse size or intensity. However, root‐zone soil moisture was significantly higher between shrubs than under shrubs. The soil moisture measured at the surface (at 5 cm depth) was not consistently different under and between shrubs, but the soil moisture measured at depths of 15 and 30 cm were both significantly higher between shrubs than under shrubs. Considering mechanisms that explain the interaction between plants and soil moisture, we found no differences in infiltration, evaporative losses, and surface soil moisture in locations under and between shrubs. This led to the conclusion that lower root‐zone soil moisture under shrubs was due largely to greater root density under shrubs than between shrubs. This study adds to the understanding of the impact of precipitation patterns on infiltration and soil moisture in shrub‐dominated sites and the potential for vegetation change in arid and semiarid lands.
Due to its modest data demands and transparent model structure, the Universal Soil Loss Equation (USLE) remains the most popular tool for water erosion hazard assessment. However, the model has ...several shortcomings, two of which are likely to have prominent implications for the model results. First, the mathematical form of the USLE, the multiplication of six factors, easily leads to large errors whenever one of the input data is misspecified. Second, the USLE has a modest correlation between observed soil losses and model calculations, even with the same data that was used for its calibration. This raises questions about its mathematical model structure and the robustness of the assumed parameter values that are implicitly assigned to the model. This paper, therefore, analyzes if the USLE could benefit from mathematical model transformations that, on one hand, mitigate the impact of incorrect input factors and, on the other hand, result in a better fit between model results and observed soil losses. For the analysis, we revisit the original data set and consider the USLE factors as variables rather than their common interpretation as parameters. We first use both nonparametric and parametric techniques to test the robustness of the implicit parameter assignments in the USLE equation. Next, we postulate alternative mathematical forms and use parametric test statistics to evaluate parameter significance and model fit. A tenfold cross-validation of the model with the best fit tests the sensitivity of the parameters for inclusion or exclusion of the data. The results show that the USLE model is not very robust, however, only slight model improvements are obtained by drastic modifications of its functional form, thereby sacrificing the simple model structure that was intended by its designers.
PDF
