Characteristics of Oxygen-18 and Deuterium Composition in Waters from the Pecos River in American Southwest

The Pecos River, situated in eastern New Mexico and western Texas, receives snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The isotopic composition of the two water sources differs from each other due to their different geographical origins in the Pacific North and the Gulf of Mexico. To better assess the physical features of the Pecos River, oxygen and hydrogen isotopic compositions (δ18O and δD), major ion concentrations, and other physical variables (e.g., water temperature and electrical conductivity) were measured on water samples collected from the main stem and its selected tributaries during a growing season (March, May, and July) in 2005. The results of this work indicate that stream water from the Pecos River contains a relatively large magnitude of variations in δ18O and δD, with δ18O ranging from − 8.9‰ to 3.6‰ and δD from − 64.5‰ to 1.6‰. The average value of δ18O is around − 3‰, which is significantly larger than that of the snowmelt but almost identical to that of the Mexican monsoonal rainfall. On the other hand, the average value of δD is around − 30‰, which is significantly larger than that of the snowmelt but lower than that of the Mexican monsoonal rainfall. Application of a dual isotope index, deuterium excess (d-excess), allows us to assess the relative contribution of various hydrologic components and processes that shape the stream hydrology of the Pecos River. The river water from the upper valley is characterized by relatively low values of δ18O and δD and relatively high values of d-excess (d = 10‰), documenting the isotopic fingerprint of the snowmelt. The middle basin is topographically gentle and its water quality has been severely affected by anthropogenic disturbances (e.g., water impoundments and diversions). As a result, chemical and isotopic composition of the water from the middle basin is highly variable, depending on its time, location, and degree of disturbances. Both δ18O and δD increase significantly from upstream to downstream and from cold to warm seasons because of evaporation-induced isotopic enrichments. The average δ18O value of the heavy-isotope-enriched waters from the middle basin is identical to that of the waters from the lower valley. In contrast, d-excess of the waters from the middle basin usually is negative, and substantially lower than that of the waters from the lower valley. Using a simple d-excess based hydrologic model, we estimated that there was up to 85% of stream flow which was derived from local freshwater sources (mainly from the Mexican monsoonal rainfall) in the lower valley and that there was up to 33% of stream water which was lost through evaporation occurring in stream channels and fields of the middle basin. Additionally, the correlation of d-excess and electrical conductivity further highlights the role of evaporative enrichments in regulating stream chemistry and isotope hydrology. This study demonstrates the usefulness of combined isotopic and geochemical data, especially the applicability of d-excess, for watershed baseline assessments

Dominant Processes Controlling Water Chemistry of the Pecos River in American Southwest

Here we show an analysis of river flow and water chemistry data from eleven gauging stations along the Pecos River in eastern New Mexico and western Texas, with time spanning 1959–2002. Analysis of spatial relationship between the long-term average flow and total dissolved solids (TDS) concentration allows us to illuminate four major processes controlling river chemistry, namely saline water addition, evaporative concentration with salt gain or loss, dilution with salt gain or loss, and salt storage. Of the 10 river reaches studied, six reaches exhibit the process dominated by evaporative concentration or freshwater dilution with little change in salt load. Four reaches show considerable salt gains or losses that are induced by surfaceground water interactions. This analysis suggests that the evaporative concentration and freshwater dilution are the prevailing mechanisms, but local processes (e.g., variations in hydrologic flowpath and lithologic formation) also play an important role in regulating the hydrochemistry of the Pecos River

Characteristics of Oxygen-18 and Deuterium Composition in Waters from the Pecos River in American Southwest

The Pecos River, situated in eastern New Mexico and western Texas, receives snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The isotopic composition of the two water sources differs from each other due to their different geographical origins in the Pacific North and the Gulf of Mexico. To better assess the physical features of the Pecos River, oxygen and hydrogen isotopic compositions (δ18O and δD), major ion concentrations, and other physical variables (e.g., water temperature and electrical conductivity) were measured on water samples collected from the main stem and its selected tributaries during a growing season (March, May, and July) in 2005. The results of this work indicate that stream water from the Pecos River contains a relatively large magnitude of variations in δ18O and δD, with δ18O ranging from − 8.9‰ to 3.6‰ and δD from − 64.5‰ to 1.6‰. The average value of δ18O is around − 3‰, which is significantly larger than that of the snowmelt but almost identical to that of the Mexican monsoonal rainfall. On the other hand, the average value of δD is around − 30‰, which is significantly larger than that of the snowmelt but lower than that of the Mexican monsoonal rainfall. Application of a dual isotope index, deuterium excess (d-excess), allows us to assess the relative contribution of various hydrologic components and processes that shape the stream hydrology of the Pecos River. The river water from the upper valley is characterized by relatively low values of δ18O and δD and relatively high values of d-excess (d = 10‰), documenting the isotopic fingerprint of the snowmelt. The middle basin is topographically gentle and its water quality has been severely affected by anthropogenic disturbances (e.g., water impoundments and diversions). As a result, chemical and isotopic composition of the water from the middle basin is highly variable, depending on its time, location, and degree of disturbances. Both δ18O and δD increase significantly from upstream to downstream and from cold to warm seasons because of evaporation-induced isotopic enrichments. The average δ18O value of the heavy-isotope-enriched waters from the middle basin is identical to that of the waters from the lower valley. In contrast, d-excess of the waters from the middle basin usually is negative, and substantially lower than that of the waters from the lower valley. Using a simple d-excess based hydrologic model, we estimated that there was up to 85% of stream flow which was derived from local freshwater sources (mainly from the Mexican monsoonal rainfall) in the lower valley and that there was up to 33% of stream water which was lost through evaporation occurring in stream channels and fields of the middle basin. Additionally, the correlation of d-excess and electrical conductivity further highlights the role of evaporative enrichments in regulating stream chemistry and isotope hydrology. This study demonstrates the usefulness of combined isotopic and geochemical data, especially the applicability of d-excess, for watershed baseline assessments

Influence of the Pacific Decadal Oscillation on Hydrochemistry of the Rio Grande, USA, and Mexico

The hydrochemistry has been examined using the major element composition of river water at 12 gauging stations along the Rio Grande. As the Rio Grande Basin consists of two watersheds that have different hydrologic and climatic regimes, two chloride concentration records from the El Paso and Falcon Dam gauging stations have been extracted to reflect long-term variability in river chemistry of the upper and lower basins over the last 50–70 years. Both records contain decadal variability in chloride concentration but are different in nature. The chloride concentration record from the upper basin displays a distinct pattern of decadal variability similar to the Pacific Decadal Oscillation (PDO). This indicates the chloride concentration at El Paso is largely determined by the amount of stream discharge of the upper basin that is associated with the PDO. Conversely, there is no such pattern of decadal variability in the chloride concentration record from the lower basin though several of the chloride concentration maxima coincide with minima in the PDO index. Instead, the chloride concentration record from the lower basin contains a progressively increasing trend of chloride concentration from 1970 to 1990, suggesting that anthropogenic disturbances (e.g., dam constructions and increased irrigation demands) may also play a role in intervening long-term changes in river chemistry

Changes in Major Element Hydrochemistry of the Pecos River in the American Southwest Since 1935

The Pecos River, situated in eastern New Mexico and western Texas, receives water from a drainage area of 91 000 km2. There are primarily two major water inputs, namely snowmelt from winter storms in the headwater region of the southern Rocky Mountains and runoff from warm-season monsoonal rainfall in the lower valley. The Pecos River suffers from high levels of total dissolved solids (TDS \u3e5000 mg L−1) under normal flow conditions. This not only poses serious problems for agricultural irrigation and safe drinking water supply, but also results in a permanent loss of biodiversity. This study examines changes in stream flow and water chemistry of the Pecos River over the last 70 a to better understand the long-term variability in stream salinity and the role of agricultural practices in salt transfer. A TDS record from the lower Pecos River near Langtry (Texas) back to 1935 was extracted to show a distinct pattern of decadal variability similar to the Pacific Decadal Oscillation (PDO), in which stream salinity is overall above average when the PDO is in positive (warm) phase and below average when the PDO is in negative (cold) phase. This is due to: (1) the dissolved salts contributed to the river are largely from dissolution of NaCl and CaSO4-bearing minerals (e.g., halite and gypsum) in the upper basin, (2) the amount of the dissolved salts that reach the lower basin is mainly determined by the stream flow yield in the upper basin and (3) the stream flow yield from the upper basin is positively correlated with the PDO index. This further attests that large-scale climatic oscillation is the major source of long-term changes in stream flow and salinity of the Pecos River. On the other hand, there is also a strong indication that the rate of salt export has been affected by reservoir operations and water diversions for agricultural practices

Salt Tolerance of Oilseed Crops during Establishment

Abstract: Bioenergy production in arid and semi-arid regions is viewed as being limited due to water resource constraints and potential competition with food production. However, there are crop rotation niches as well as opportunities for utilizing saline soils and water which are not conducive to high value crop production. An exploratory study was thus conducted in a greenhouse for assessing salt tolerance of various oilseed crops during establishment, which is often the critical stage for successful production. Canola (Brassica napus), and safflower (Carthamus tinctorius) were salt-tolerant during germination, but emergence was curtailed owing to soil crusting. Camelina (C. sativa) germinated well, yet emergence was poor, probably due to weak hypocotyl. Field pennycress (Thlaspi arvense) and Lesquerella (L. fendleri) were salt sensitive, and could not germinate in NaCl solutions higher than 50 mM. Salicornia (S. bigelovii), a halophyte, is extremely salt-tolerant once established, but not during seedling emergence. Seedling emergence which had little correlation with salt tolerance of established plants, was constrained not only by salt tolerance at germination and the pattern of salt accumulation at the soil surface, but also by seed size and soil crust development. At the present state of field management capability, safflower and canola are among the most promising oilseed crops for saline areas, and can potentially be grown as a winter rotation crop with a comparatively low water requirement. For the species with small seed, crop improvements towards greater seedling vigor as well as the effective establishment methods have to be developed