Jorge A. Ramírez

The complementary relationship in estimation of regional evapotranspiration: An enhanced advection-aridity model

Long-term monthly evapotranspiration estimates from Brutsaert and Strickers Advection-Aridity model were compared with independent estimates of evapotranspiration derived from long-term water balances for 139 undisturbed basins across the conterminous United States. On an average annual basis for the period 1962-1988 the original model, which uses a Penman wind function, underestimated evapotranspiration by 7.9% of precipitation compared with the water balance estimates. Model accuracy increased with basin humidity. An improved formulation of the model is presented in which the wind function and the Priestley-Taylor coefficient are modified. The wind function was reparameterized on a seasonal, regional basis to replicate independent proxy potential evapotranspiration surfaces. This led to significant differences from the original Penman wind function. The reparameterized wind function, together with a recalibrated Priestley- Taylor coefficient in the wet environment evapotranspiration formulation, reduced the underestimation of annual average evapotranspiration to only 1.15% of precipitation on an independent set of validation basins. The results offered here lend further support for Bouchets hypothesis as it applies to large-scale, long-term evapotranspiration.

The complementary relationship in estimation of regional evapotranspiration: the Complementary Relationship Areal Evapotranspiration and Advection-Aridity models

Two implementations of the complementary relationship hypothesis for regional evapotranspiration, the Complementary Relationship Areal Evapotranspiration (CRAE) model and the Advection-Aridity (AA) model, are evaluated against independent estimates of regional evapotranspiration derived from long-term, large-scale water balances (1962-1988) for 120 minimally impacted basins in the conterminous United States. The CRAE model overestimates annual evapotranspiration by 2.5% of mean annual precipitation, and the AA model underestimates annual evapotranspiration by 10.6% of precipitation. Generally, increasing humidity leads to decreasing absolute errors for both models, and increasing aridity leads to increasing overestimation by the CRAE model and underestimation by the AA model, with the exception of high, arid basins, where the AA model overestimates evapotranspiration. Overall, the results indicate that the advective portion of the AA model must be recalibrated before it may be used successfully on a regional basis and that the CRAE model accurately predicts monthly regional evapotranspiration.

A numerical method for simulating discontinuous shallow flow over an infiltrating surface

SUMMARY A numerical method based on the MacCormack finite difference scheme is presented. The method was developed for simulating two-dimensional overland flow with spatially variable infiltration and microtopography using the hydrodynamic flow equations. The basic MacCormack scheme is enhanced by using the method of fractional steps to simplify application; treating the friction slope, a stiff source term, point-implicitly, plus, for numerical oscillation control and stability, upwinding the convective acceleration term. A higher-order smoothing operator is added to aid oscillation control when simulating flow over highly variable surfaces. Infiltration is simulated with the Green‐Ampt model coupled to the surface water component in a manner that allows dynamic interaction. The developed method will also be useful for simulating irrigation, tidal flat and wetland circulation, and floods. Copyright

Observational evidence of the complementary relationship in regional evaporation lends strong support for Bouchet's hypothesis

Using independent observations of actual and potential evapotranspiration at a wide range of spatial scales, we provide direct observational evidence of the complementary relationship in regional evapotranspiration hypothesized by Bouchet in 1963. Bouchet proposed that, for large homogeneous surfaces with minimal advection of heat and moisture, potential and actual evapotranspiration depend on each other in a complementary manner through land-atmosphere feedbacks. Although much work has been done that has led to important theoretical and conceptual insights about regional actual evapotranspiration and its relation to regional potential evapotranspiration, never before has a data set of direct observations been assembled that so clearly displays complementarity, providing strong evidence for the complementary relationship hypothesis, and raising its status above that of a mere conjecture.

Topographic, meteorologic, and canopy controls on the scaling characteristics of the spatial distribution of snow depth fields

[1] In this study, LIDAR snow depths, bare ground elevations (topography), and elevations filtered to the top of vegetation (topography + vegetation) in five 1-km 2 areas are used to determine whether the spatial distribution of snow depth exhibits scale invariance, and the control that vegetation, topography, and winds exert on such behavior. The one-dimensional and mean two-dimensional power spectra of snow depth exhibit power law behavior in two frequency intervals separated by a scale break located between 7 m and 45 m. The spectral exponents for the low-frequency range vary between 0.1 and 1.2 for the one-dimensional spectra, and between 1.3 and 2.2 for the mean twodimensional power spectra. The spectral exponents for the high-frequency range vary between 3.3 and 3.6 for the one-dimensional spectra, and between 4.0 and 4.5 for the mean two-dimensional spectra. Such spectral exponents indicate the existence of two distinct scaling regimes, with significantly larger variations occurring in the larger-scale regime. Similar bilinear power law spectra were obtained for the fields of vegetation height, with crossover wavelengths between 7 m and 14 m. Further analysis of the snow depth and vegetation fields, together with wind data, support the conclusion that the break in the scaling behavior of snow depth is controlled by the scaling characteristics of the spatial distribution of vegetation height when snow redistribution by wind is minimal and canopy interception is dominant, and by the interaction of winds with features such as surface concavities and vegetation when snow redistribution by wind is dominant.

Recent Trends in Precipitation and Streamflow in the Rio Puerco Basin

Abstract River systems in semiarid regions are susceptible to rapid and dramatic channel erosion and arroyo formation. Climate plays an important role in arroyo development through changes in precipitation intensity, seasonality, and variability. Here, trends in precipitation and streamflow at the annual, monthly, and daily timescales for the last 50 yr are analyzed for the Rio Puerco Basin in northwestern New Mexico, and connections with recent watershed and channel changes are examined. The increasing trend in annual precipitation in the basin is shown to be part of larger-scale climatic variability that affects the U.S. Southwest region, which is associated with climatic anomalies in the northern Pacific. Results of hydroclimatic data analyses point to a general increase in wetness in nonsummer months—an increase in the number of rainy days and in the frequency of flow days in the stream system is observed. There are substantial shifts in the distributions of both daily precipitation and streamflow. Ra...

Energy dissipation theories and optimal channel characteristics of river networks

The effects of energy dissipation on channel properties of a river network are explored. On the basis of a local and global hypothesis of optimality in energy expenditure, we investigate the relationships between channel hydraulic geometry, flow velocity, channel bed slope, and streamflow conditions in optimal river networks. Expressions for the rate of energy dissipation per unit channel area Pa are derived as functions of cumulative drainage area and river network parameters. Optimal channel characteristics are developed that satisfy the hypothesis of local optimality, and provide constant Pa throughout the river network. We show that these optimal channel characteristics are remarkably similar to those of many natural river systems in their downstream hydraulic geometry exponents, channel bed slope scaling, spatial distribution of average flow velocity, boundary shear, resistance to flow, etc. Optimal combinations of channel downstream hydraulic geometry and basin topography were analyzed on data from Goodwin Creek. We found ranges of optimality for the combination of the downstream hydraulic geometry exponent for width of Leopold and Maddock [1953] (0.32 < b < 0.74), and the channel bed slope scaling exponent (−0.65 < z < −0.29), and argue that river networks develop average channel properties within these ranges in order to attain constant Pa throughout the network. We propose that the hypothesis of local optimality is a central principle that explains the average behavior and adjustment of channel characteristics in natural river systems.

A Statistical–Dynamical Parameterization of Interception and Land Surface–Atmosphere Interactions

At a local scale, the interception capacity of the canopy depends on a variety of climatic and canopy factors. Of particular importance is the intensity of rainfall—interception capacity varies inversely with rainfall intensity. At a field or regional scale, like the scale of global climate models, the spatially averaged interception also depends significantly on the spatial variability of rainfall intensity and total precipitation depth. A new parameterization of canopy interception is developed. In the new parameterization, the spatial average of actual interception is obtained as a function of rainfall intensity and total precipitation depth, and of an interception capacity, which depends on the characteristics of the leaf surface and of the vegetation cover. In a statistical‐dynamical framework, the new parameterization also accounts for the subgrid-scale spatial variability of rainfall intensity and total precipitation depth. The implications of accounting for the dependence of interception capacity on rainfall characteristics are examined by assessing the consequent responses of the energy and the water fluxes at the land surface. This is accomplished by incorporating the new parameterization into a soil‐plant‐atmosphere column model that is fundamentally based on the physical parameterizations of NCAR’s Community Climate Model.

On downstream hydraulic geometry and optimal energy expenditure: case study of the Ashley and Taieri Rivers

Abstract The downstream distribution of channel geometry and of the rate of energy expenditure per unit channel area P a are analyzed on extensive datasets from the Ashley and Taieri Rivers in New Zealand. We investigate whether the rivers conform to local optimality, by which P a tends to be constant throughout the network. We look at energy expenditure from two perspectives. (1) In the context of downstream hydraulic geometry (DHG) relations, we derive equations for the general unconstrained optimal combination of the DHG exponents b and f under local optimality, and compare these with data-derived exponents from the Ashley and Taieri Rivers. (2) Treating P a as a random variable, we examine the downstream scaling of moments of P a . Results suggest that E ( P a ) is constant in the higher order stream network in both basins. The scaling of Var( P a ) with discharge illustrates that basin and channel heterogeneity play an important role in understanding the spatial distribution of energy expenditure and its variability in river systems.

AN ANALYSIS OF ENERGY EXPENDITURE IN GOODWIN CREEK

The local optimality hypothesis that natural river systems adjust their average channel properties toward an optimal state in which the rate of energy dissipation per unit channel area, Pa, is constant throughout the river network is explored on an analysis of Goodwin Creek, Mississippi. River network parameters describing the variation of channel forming and maintaining discharge, channel downstream hydraulic geometry, bed slope, and sediment concentration as a function of cumulative drainage area are estimated from Goodwin Creek data. Optimal channel characteristics that produce constant P a are determined and superposed onto the digital elevation model- extracted river network with reach averaged bed slopes, and the spatial distribution of the energy dissipation rate Pa throughout the network is analyzed. Channel reaches with average energy dissipation rates different from the constant value of the optimal network are identified. We argue that these reaches are potentially unstable relative to the remainder of the network, and that their average channel properties will adjust in the direction of constant Pa. Qualitative statements are made about the direction of this adjustment through differences between the observed and optimal channel widths, and comparisons are made with recent observations of channel change in Goodwin Creek. This energy expenditure analysis suggests that the hypothesis of local optimality can be a useful tool for studying the relative stability and potential channel adjustment of river networks.