Thomas Maddock

Seasonal estimates of riparian evapotranspiration using remote and in situ measurements

In many semi-arid basins during extended periods when surface snowmelt or storm runoff is absent, groundwater constitutes the primary water source for human habitation, agriculture and riparian ecosystems. Utilizing regional groundwater models in the management of these water resources requires accurate estimates of basin boundary conditions. A critical groundwater boundary condition that is closely coupled to atmospheric processes and is typically known with little certainty is seasonal riparian evapotranspiration (ET). This quantity can often be a significant factor in the basin water balance in semi-arid regions yet is very difficult to estimate over a large area. Better understanding and quantification of seasonal, large-area riparian ET is a primary objective of the Semi-Arid Land-Surface-Atmosphere (SALSA) Program. To address this objective, a series of interdisciplinary experimental campaigns were conducted in 1997 in the San Pedro Basin in southeastern Arizona. The riparian system in this basin is primarily made up of three vegetation communities: mesquite (Prosopis velutina), sacaton grasses (Sporobolus wrightii), and a cottonwood (Populus fremontii)/willow (Salix goodingii) forest gallery. Micrometeorological measurement techniques were used to estimate ET from the mesquite and grasses. These techniques could not be utilized to estimate fluxes from the cottonwood/willow (C/W) forest gallery due to the height (20‐30 m) and non-uniform linear nature of the forest gallery. Short-term (2‐4 days) sap flux measurements were made to estimate canopy transpiration over several periods of the riparian growing season. Simultaneous remote sensing measurements were used to spatially extrapolate tree and stand measurements. Scaled C/W stand level sap flux estimates were utilized to calibrate a Penman‐Monteith model to enable temporal extrapolation between synoptic measurement periods. With this model and set of measurements, seasonal riparian vegetation water use estimates for the riparian corridor were obtained. To validate these models, a 90-day pre-monsoon water balance over a 10 km section of the river was carried out. All components of the water balance, including riparian ET, were

The water use of two dominant vegetation communities in a semiarid riparian ecosystem

Consumptive water use from riparian evapotranspiration is a large component of many semiarid basins’ groundwater budgets — comparable in magnitude to mountain front recharge and surface water discharge. In most long-term groundwater studies the amount of water used by phreatophytes is estimated by empirical formulae and extrapolation of measurements taken elsewhere. These approaches are problematic due to the uncertainties regarding the vegetation’s water source (e.g., groundwater or recent precipitation) and its magnitude. Using micrometeorological techniques in this study, surface energy and water fluxes were measured for an annual cycle over two dominant types of vegetation in the riparian floodplain of the San Pedro River in southeastern Arizona. The vegetation communities were a perennial, floodplain sacaton grassland (Sporobolus wrightii) and a tree/shrub grouping composed largely of mesquite (Prosopis velutina). These measurements are compared with estimates from previous studies. Additionally, measurements of soil water content and water table levels are used to infer the dominant sources of the evaporated water. The results indicate that the grassland relied primarily on recent precipitation, while the mesquite obtained water from deeper in the soil profile. Neither appears to be strongly phreatophytic, which suggests that the dominant, natural groundwater withdrawals in the Upper San Pedro Basin are mainly confined to the narrow cottonwood/willow gallery that lines the river.

Preface paper to the Semi-Arid Land-Surface-Atmosphere (SALSA) Program special issue.

The Semi-Arid Land-Surface-Atmosphere Program (SALSA) is a multi-agency, multi-national research effort that seeks to evaluate the consequences of natural and human-induced environmental change in semi-arid regions. The ultimate goal of SALSA is to advance scientific understanding of the semi-arid portion of the hydrosphere-biosphere interface in order to provide reliable information for environmental decision making. SALSA approaches this goal through a program of long-term, integrated observations, process research, modeling, assessment, and information management that is sustained by cooperation among scientists and information users. In this preface to the SALSA special issue, general program background information and the critical nature of semi-arid regions is presented. A brief description of the Upper San Pedro River Basin, the initial location for focused SALSA research follows. Several overarching research objectives under which much of the interdisciplinary research contained in the special issue was undertaken are discussed. Principal methods, primary research sites and data collection used by numerous investigators during 1997-1999 are then presented. Scientists from about 20 US, five European (four French and one Dutch), and three Mexican agencies and institutions have collaborated closely to make the research leading to this special issue a reality. The SALSA Program has served as a model of interagency cooperation by breaking new ground in the approach to large scale interdisciplinary science with relatively limited resources.

Seasonalizing mountain system recharge in semi-arid basins-climate change impacts.

Climate variability and change impact groundwater resources by altering recharge rates. In semi-arid Basin and Range systems, this impact is likely to be most pronounced in mountain system recharge (MSR), a process which constitutes a significant component of recharge in these basins. Despite its importance, the physical processes that control MSR have not been fully investigated because of limited observations and the complexity of recharge processes in mountainous catchments. As a result, empirical equations, that provide a basin-wide estimate of mean annual recharge using mean annual precipitation, are often used to estimate MSR. Here North American Regional Reanalysis data are used to develop seasonal recharge estimates using ratios of seasonal (winter vs. summer) precipitation to seasonal actual or potential evapotranspiration. These seasonal recharge estimates compared favorably to seasonal MSR estimates using the fraction of winter vs. summer recharge determined from isotopic data in the Upper San Pedro River Basin, Arizona. Development of hydrologically based seasonal ratios enhanced seasonal recharge predictions and notably allows evaluation of MSR response to changes in seasonal precipitation and temperature because of climate variability and change using Global Climate Model (GCM) climate projections. Results show that prospective variability in MSR depends on GCM precipitation predictions and on higher temperature. Lower seasonal MSR rates projected for 2050-2099 are associated with decreases in summer precipitation and increases in winter temperature. Uncertainty in seasonal MSR predictions arises from the potential evapotranspiration estimation method, the GCM downscaling technique and the exclusion of snowmelt processes.

Comparison of riparian evapotranspiration estimates based on a water balance approach and sap flow measurements

Estimates of evapotranspiration (ET) from riparian vegetation along a 122 m reach of the San Pedro River using both a water balance approach and by scaling up sap flow measurements are compared. A sensitivity analysis was performed on the components of the water balance to assess the effects of measurement errors on estimates of ET using this method. It was concluded that by reducing the error in three key components to less than 5%, riparian ET could be estimated to an accuracy of 20-25% using the water balance method. The analysis also indicated that random measurement errors up to 10% in the water balance measurements would explain the difference between the water balance and sap flow ET estimates. Demonstrating agreement given reasonable error bounds provides confidence in the accuracy of both methods.

Drawdown, Velocity, Storage, and Capture Response Functions for Multiaquifer Systems

Response functions for drawdown, velocity, storage losses, and capture describe the spatial and temporal reaction of an aquifer to a unit pumping stress. This paper extends the use of response functions to the notion of capture for multilayered aquifer systems. Capture describes the pumping-induced quantity of water gained by the aquifer from internal or boundary sources. Internal sources include rivers and hydraulically connected aquifers. Boundary sources include constant head and head-dependent boundaries. Although response functions are defined only for linear systems, a methodology is demonstrated for implementing response functions in cases of nonlinear capture. The value of response functions lies in their utility as hydrologic constraints in management optimization models. An example demonstrates the use of separable programming to determine the nonlinear effects of drawdown below the bottom of a streambed.

An optimum reduction of gauges to meet data program constraints

ABSTRACT Budget or manpower constraints may force a reduction in data collection activities. However, information may be transferred from continued gauge sites to discontinued gauge sites provided there is ‘sufficient’ correlation between flow sequences. Information defined as the reciprocal of variance (of the parameter estimator for which the gauge has been established) is provided at ungauged sites by regression techniques. A set of gauge sites is chosen to satisfy the budget constraint and to maximize the information content. The information content based on regression is a function of the correlations between flow sequences and is subject to the errors in their estimators. Post optimal analysis gives estimates of correlation error effects on the decision process as to which gauges to continue and which to discontinue.

RIPGIS-NET: a GIS tool for riparian groundwater evapotranspiration in MODFLOW.

RIPGIS-NET, an Environmental System Research Institute (ESRIs) ArcGIS 9.2/9.3 custom application, was developed to derive parameters and visualize results of spatially explicit riparian groundwater evapotranspiration (ETg), evapotranspiration from saturated zone, in groundwater flow models for ecohydrology, riparian ecosystem management, and stream restoration. Specifically RIPGIS-NET works with riparian evapotranspiration (RIP-ET), a modeling package that works with the MODFLOW groundwater flow model. RIP-ET improves ETg simulations by using a set of eco-physiologically based ETg curves for plant functional subgroups (PFSGs), and separates ground evaporation and plant transpiration processes from the water table. The RIPGIS-NET program was developed in Visual Basic 2005, .NET framework 2.0, and runs in ArcMap 9.2 and 9.3 applications. RIPGIS-NET, a pre- and post-processor for RIP-ET, incorporates spatial variability of riparian vegetation and land surface elevation into ETg estimation in MODFLOW groundwater models. RIPGIS-NET derives RIP-ET input parameters including PFSG evapotranspiration curve parameters, fractional coverage areas of each PFSG in a MODFLOW cell, and average surface elevation per riparian vegetation polygon using a digital elevation model. RIPGIS-NET also provides visualization tools for modelers to create head maps, depth to water table (DTWT) maps, and plot DTWT for a PFSG in a polygon in the Geographic Information System based on MODFLOW simulation results.

Conjunctive Water Management in the US Southwest

Water demands in the US Southwest have been subject to great pressures due to explosive population growth and climate variability that has produced decadal droughts. These pressures have led to unsustainable use of surface water and groundwater, forcing states to adopt conjunctive management of ground and surface water systems. Unfortunately, federal and state laws have not kept pace with the scientific development of management strategies. A series of examples are presented to illustrate some successes and failures of integration of surface water and groundwater management and its accompanying legal implications.

The Upper San Pedro River Basin

The San Pedro River begins in Sonora, Mexico, and flows northward through Arizona, United States, before joining the Gila River, which flows into the Colorado, and finally empties into the Gulf of California.… The Upper San Pedro River Basin in Sonora and Arizona is the focus of a number of urgent, complex, interrelated, and controversial issues, including its international importance as bird habitat, its attractiveness to development, and the vulnerability of its landscape to changes caused directly by development and indirectly via continued lowering of the groundwater table.