Semi-arid Southern High Plains Research Articles

The Performance of an Alfalfa–Sainfoin Mixture in the Semiarid Southern High Plains of the USA

While alfalfa (Medicago sativa) is widely used as forage because of its many positive attributes, there are concerns regarding bloat in grazing ruminants, but tannin-containing legumes, such as sainfoin (Onobrychis viciifolia), are non-bloating. Based on interest elsewhere for growing alfalfa–sainfoin mixtures to address bloat concerns, the objectives of a study conducted at Tucumcari, NM, USA, were to compare an alfalfa–sainfoin mixture to the respective monocultures for stand persistence and dry matter yield over three production years in three randomized complete blocks. The onset of drought limited irrigation water availability shortly after the study’s initiation, which limited sainfoin persistence to the second year after seeding. The resulting percentage decline in forage mass in June from year 1 to year 2 was alfalfa > alfalfa–sainfoin > sainfoin. That said, the alfalfa mass was considerably greater than the sainfoin mass, and the June mass of both monocultures was consistent with that in previous studies in the region when fully irrigated (5.57 vs. 2.00 Mg ha−1 for monoculture alfalfa and sainfoin, respectively; p < 0.0004). Alfalfa persistence was greater than that of sainfoin after 3 years. Alfalfa–sainfoin mixtures may have value in the Southern High Plains of the USA and similar semiarid environments when fully irrigated. Additional studies should evaluate mixture seeding ratios and stand performance under fully irrigated conditions with sainfoin reseeding.

Diet specialization mediates drivers of Cucurbita herbivory in a semi-arid agroecosystem

Herbivory is a major fitness pressure for plants and a key driver of crop losses in agroecosystems. Dense monocultures are expected to favor specialist herbivorous insects, particularly those who primarily consume crop species; yet, levels and types of herbivory are not uniform within regional cropping systems. It is essential to determine which local and regional ecological factors drive variation in herbivory in order to support functional agroecosystems that rely less on chemical inputs. Crops in the genus Cucurbita host a suite of both generalist and specialist herbivores that inflict significant damage, yet little is known about the relative contribution of these herbivores to variation in herbivory and how local- and landscape-scale Cucurbita resource concentrations, management practices, and natural enemies mediate this relationship. In this study, we tested whether three foundational ecological hypotheses influenced Cucurbita herbivory across 20 pumpkin fields in the semi-arid Southern High Plains Region of Texas. We used generalized linear mixed models and confirmatory path analysis to assess whether the Density-dependent Herbivory Hypothesis, Resource Concentration Hypothesis, or the Natural Enemies Hypothesis, could explain variation in Cucurbita herbivory and insect dynamics in the context of conventional agronomic practices. We found that herbivory increased over time, indicating that herbivores were causing sustained damage throughout the growing season. We also found that fields with higher local Cucurbita resources had lower herbivory, suggesting a resource dilution effect. Natural enemy communities were more abundant and taxonomically rich in sites with greater generalist herbivore abundance, though predator abundance declined over time, indicating that late-season crop fields are most at risk given high herbivory and low natural enemy-based control. Our findings also suggest that while local resource availability may drive the abundance and richness of arthropod communities, additional agronomic and phenological information is needed to anticipate herbivory risk in an agriculturally dominated landscape.

Semi-arid soil bacterial communities are refined by altered plant selection pressure under conservation management practices

Plants' ability to select for, and refine, their microbiome over time is well known. In soil, the strength of plant selection pressure on microbial composition can result in distinct rhizosphere, and root-zone communities, but may be masked in soils with lower native diversity such as semi-arid soils. Semi-arid soils are also known for their high potential for wind erosion, which may be mitigated by the implementation of cover crops. These cover crops may also alter the microbial composition of the semi-arid soils, but the strength of this effect, and the resulting changes in the composition of the microbiome, are less well known. This study aimed to address whether the implementation of a single-species wheat cover crop in semi-arid agricultural soils would impact the bacterial composition of the cash crop. We investigated three tillage regimes—no-tillage with a winter wheat cover crop, no-tillage winter fallow, and conventional tillage winter fallow—to determine the effects of tillage removal, and cover crop implementation on the soil bacterial community present in the root zone of the cash crop. It was determined that the composition of the bacterial microbiome was similar between cover cropped and non-cover cropped treatments. The similarity of bacterial composition was likely due to the strong effect of environmental selection reducing overall biodiversity in these soils. However, the variability of bacterial community composition was strongly reduced for the cover cropped treatment compared to the two non-cover cropped treatments. The similarity of bacterial communities with cover cropping, without significant soil physicochemical changes, suggests that plant-selection pressure, rather than management-induced changes in soil physicochemical properties, drove the refinement (decreased dissimilarity compared to non-cover cropped treatments) of the bacterial community. The composition of this refined community under cover cropping was determined to contain an enrichment of several genera of interest including N cycling and potential plant-growth promoting bacteria. In summary, the introduction of cover cropping in the nutrient and microbial-poor soil of the semi-arid Southern High Plains strongly refined the bacterial community and selected for organisms with the potential to improve soil functions associated with greater agronomic sustainability.

Nitrogen fertilizer driven nitrous and nitric oxide production is decoupled from microbial genetic potential in low carbon, semi-arid soil

IntroductionNitrous oxide (N2O) emission from soil is a major concern due to its contribution to global climate change and its function as a loss mechanism of plant-available nitrogen (N) from the soil. This is especially true in intensive agricultural soils with high rates of N fertilizer application such as those on the semi-arid Southern High Plains, USA.MethodsThis study examined emissions of N2O, pore-space concentrations of N2O and nitric oxide (NO), soil chemical properties, water content, and the genetic potential for N cycling five years after conservation system and N management implementation.ResultsFor these semi-arid soils with low N, carbon, and water contents, large soil N2O emissions (up to 8 mL N2O-N m-2 day-1) are directly related to the application of N fertilizer which overwhelms the N2O reducing capacity of the soil. When this fertilizer N is depleted, N2O flux is either low, non-existent, or net-negative and has been observed as early as mid-season for preplant applied N fertilizer (-0.1 mL N2O-N m-2 day-1). Soil pore-space gas concentrations (N2O and NO) remained relatively constant across the growing season (average N2O: 0.78 µL N2O L-1 soil air; NO: 3.3 µL NO L-1 soil air, indicating a base-level of N-cycle activity, but was not directly related to surface emissions of N2O which decreased across the growing season. In addition, genetic potential for N cycle activities increased across the growing season simultaneously with stagnant/reduced N cycle activity. This reflects the difficulty in relating genetic potential to in-situ activity in field research.ConclusionIt is likely that in a nutrient and carbon-poor soil, such as the semi-arid agricultural soil in this study, the microbial processes associated with N cycling are mostly limited by inorganic-N and less directly related to genetic potential at the time of sampling.

Preliminary crop coefficients for late planted short‐season soybean: Texas High Plains

AbstractHaving similar profit potential but roughly half of the water requirements of corn (Zea mays L.), cotton (Gossypium hirsutum L.) has become an attractive crop in the Southern High Plains (SHP) where groundwater levels and well capacities continue to decline. However, unpredictable and erratic precipitation often results in conditions either too wet or too dry for ideal germination. Coupled with a relatively short planting window for adequate heat unit accumulation and the threat of severe weather events, cotton crops can fail, leaving limited options for producers wanting to plant a second summer crop. One option is short‐season soybean [Glycine max (L.) Merr.]. However, limited data on crop water use (evapotranspiration, ET), crop coefficient (Kc), and yield exist for late‐planted soybean in the semiarid SHP. After unseasonably wet conditions in May 2019, a short‐season soybean variety was planted on 13 June in irrigated weighing lysimeter fields at the USDA‐ARS Conservation and Production Research Laboratory at Bushland, TX. Results showed profitable yields (4,786–4,996 kg ha–1), and although maximum daily Kc values were not different than published values, season length was 24–29 d shorter than for soybean planted in mid‐May. Seasonal water use was less than that of archived data using earlier planting dates in 2 of 3 yr, although those studies used different varieties. Crop coefficient functions using both short and tall standardized reference ET values are presented for sprinkler and subsurface drip irrigation systems. Although these data are useful to producers in the SHP, additional data are needed to develop Kc functions that better characterize interannual variations in weather.

Spring safflower water use patterns in response to preseason and in-season irrigation applications

The deep root system of safflower (Carthamus tinctorius L.) is an important adaptive trait under rainfed conditions in semi-arid agriculture. Lower water requirements and ability to tolerate abiotic stresses make safflower a potential alternative crop for the Southern High Plains (SHP). However, information on water extraction patterns of safflower and its role in oil yield formation under center pivot irrigation system that keeps only the surface soil profile wet with frequent irrigations is limited. Therefore, a field experiment was conducted in a split-plot design at Clovis, New Mexico during 2012 and 2013 growing seasons to assess the soil water use patterns, evapotranspiration, and oil yield of two spring safflower cultivars under different preseason and in-season irrigation levels. Half of the experimental units received 164 and 153 mm of preseason irrigation in 2012 and 2013, respectively, while the other half remained unirrigated. Five in-season irrigation levels (I1–I5) ranging from 88 to 392 mm in 2012 and from 83 to 373 mm in 2013 were applied to preseason and no-preseason irrigation blocks. On average, safflower extracted 70 and 28 mm of stored soil moisture in the preseason and no-preseason irrigation treatments, which significantly increased evapotranspiration (ET), water use efficiency (WUE), and oil yield in preseason compared to no-preseason irrigation treatment. Average water extraction decreased with increasing level of in-season irrigation and a decline of 32, 27, and 25% in soil moisture at the end of the season compared to initial soil moisture were observed in I1, I3, and I5, respectively. Since, WUE was not much affected by in-season irrigation treatment in both years, limiting the in-season irrigation level to I4 can save 17% of irrigation with a corresponding 9% reduction in oil yield. The relatively smaller differences were observed for water extraction and WUE among the cultivars. Overall, safflower was able to use preseason irrigation water more efficiently in semi-arid SHP. Growers can pre-irrigate safflower and reduce its in-season irrigation to allocate more water to high water requirement traditional-crops such as corn.

Growth and Yield of Guar (<i>Cyamopsis tetragonoloba</i> L.) Genotypes under Different Planting Dates in the Semi-Arid Southern High Plains

Guar is a drought and salt tolerant summer annual legume, which could be a potential alternative crop in the semi-arid Southern High Plains. Increased use of guar gum in oil industries has increased the demand of guar globally. Planting date effects on stand establishment, physiological parameters, and yield formation of guar genotypes were investigated at the New Mexico State University’s Agricultural Science Center at Clovis, NM for two seasons (2014 and 2015). Four guar genotypes (HES 1123, Kinman, Lewis, and Matador) were tested under three planting dates (June 18, July 7, and July 22 in 2014; and June 18, July 6, and July 20 in 2015). Higher temperature and rainfall were recorded under mid-June planting than early-July and late-July plantings. Guar planted under mid-June had better stand establishment as shown by the higher number of plants m-2, better physiology as revealed by higher photosynthetic rate (Pn), transpiration rate (Tr), leaf area index (LAI), and SPAD values than early-July and late-July plantings. Guar planted under mid-June resulted in taller plants, and therefore, produced higher plant biomass than both of the July plantings. Yield attributing characteristics including clusters plant-1, pods plant-1, seeds plant-1, seed spod-1, 1000 seed weight, and harvest index (HI) were highest under mid-June planting followed by the early-July and late-July plantings, respectively. The mid-June planting increased seed yield by 26% and 55% over early-July and late-July (1399 vs. 1111 and 903 kg·ha-1) plantings, respectively in 2014; while the same increase in 2015 was 51% and 243% (1308 vs. 868 and 381 kg·ha-1), respectively. These results indicate that delaying planting beyond mid-June is detrimental to guar productivity. However, genotypes did not show any significant variation in their performance. Overall, warmer growing conditions and more precipitation under mid-June planting caused better growth and yield formation of guar genotypes.

Drought response and yield formation of spring safflower under different water regimes in the semiarid Southern High Plains

Safflower (Carthamus tinctorius L.) is a deep rooted drought tolerant crop that originated in desert environments of the Middle East, and could be very well adapted to the semi-arid Southern High Plains. A field experiment was conducted at Clovis, New Mexico during 2012 and 2013 seasons to assess drought physiology and yield formation of two diverse spring safflower cultivars under different irrigation levels with or without preseason irrigation. One half of the experimental blocks received preseason irrigation of 164mm in 2012 and 153mm in 2013 to refill the soil profile utilized by the previous crops, while the other half remained depleted. Five in-season irrigation levels (I1–I5) ranging from 88 to 392mm in 2012 and from 83 to 373mm in 2013 were imposed on both preseason irrigation and no-preseason irrigation blocks. Higher leaf water potential (Ψl) was observed under increased water availability either by preseason irrigation or by higher in-season irrigation level in safflower during two observation dates in both years. Osmotic potential at full turgor (Ψπ100), photosynthesis rate (Pn) and transpiration rate (Tr) decreased with a reduction in Ψl under water stress conditions. The relative water content (RWC) was affected only by the in-season irrigation levels in both years. The preseason irrigation increased seed yield of safflower by 39 and 118% over no-preseason irrigation in 2012 and 2013, respectively. A gradual increase in seed yield was observed with an increase in irrigation levels; and the highest irrigation level, I5 increased seed yield by 85 and 171% over the lowest irrigation level, I1 in 2012 and 2013, respectively. Seed yield increased with increase in Pn, plant biomass, number of heads per plant, and number of seeds per head but not with 1000-seed weight under increased water availability. Overall, increased availability of water through preseason irrigation or through in-season irrigation levels improved safflower physiology and yield formation.

Biomass and Cellulosic Ethanol Production of Forage Sorghum Under Limited Water Conditions

This study presents results from a 2-year evaluation of biomass and cellulosic ethanol (EtOH) production potential of forage sorghum (Sorghum bicolor L. Moench) cultivars differing in brown midrib trait (i.e., bmr12) under dryland (no irrigation) and limited irrigation (2.88 mm day−1; subsurface drip) in the semiarid Southern High Plains of the USA. Commercial cultivar Sorghum Partners 1990 (SP 1990, conventional non-bmr) produced significantly more biomass (29–62 %) than a bmr12 cultivar PaceSetter bmr (PS bmr) under irrigated and dryland conditions during both years of this study. However, PS bmr biomass had higher cellulosic EtOH conversion efficiency than SP 1990 in both years according to simultaneous saccharification and fermentation analysis. Irrigation resulted in 26–49 % more biomass and 28–72 % more cellulosic EtOH production during both growing seasons, indicating that limited irrigation had favorable effects on both biomass and biofuel production. In the first year, when precipitation was below average, both cultivars produced similar amounts of cellulosic EtOH. During the second year, when precipitation was above average, higher biomass production of SP 1990 resulted in 28 % higher cellulosic EtOH production than PS bmr when averaged across both irrigated and dryland conditions. The large range of cellulosic EtOH production (1,600 to 3,380 L ha−1) during the 2 years of this study was primarily driven by differences in water availability that resulted from precipitation and irrigation. Our findings indicates that chemical composition and biomass yield potential of sorghum cultivars are critical factors that affect biomass and biofuel production under limited water conditions.

Early changes due to sorghum biofuel cropping systems in soil microbial communities and metabolic functioning

Evaluation of biofuel production cropping systems should address not only energy yields but also the impacts on soil attributes. In this study, forage sorghum (Sorghum bicolor L. Moench) cropping systems were initiated on a low organic matter soil (<0.9 %) with a history of intensively tilled low-input cotton production in the semiarid Southern High Plains of the U.S. Sorghum cropping systems were evaluated in a split-plot design with sorghum cultivar as the main plot and the combination of irrigation level (non-irrigated and deficit irrigated) and aboveground biomass removal rate (50 % and 100 %) as the split plot. The sorghum cultivars used varied in yield potential and lignin content, which are important features for feedstock-producing crops. Within 1 year, the transition from long-term cotton cropping systems to sorghum biofuel cropping systems resulted in increased soil microbial biomass C (16 %) and N (17 %) and shifts in the microbial community composition as indicated by differences in fatty acid methyl ester (FAME) profiles. Additionally, enzyme activities targeting C, N, P and S cycles increased 15–75 % (depending on the enzyme) after two growing seasons. Increased enzyme activities (16–19 %) and differences in FAME profiles were seen due to irrigation regardless of aboveground biomass removal rate. Biomass removal rate and the cultivar type had little effect on the soil microbial properties during the time frame of this study. Early results from this study suggest improvements in soil quality and the sustainability of sorghum biofuel cropping for low organic matter agricultural soils.

ET Mapping with High-Resolution Airborne Remote Sensing Data in an Advective Semiarid Environment

AbstractAccurate estimates of spatially distributed evapotranspiration (ET) are essential for managing water in irrigated regions and for hydrologic modeling. METRIC (Mapping ET at high Resolutions with Internal Calibration) energy balance algorithm was applied to derive ET from six high-resolution aircraft images (0.5–2.0 m pixels). Images were acquired over the USDA Agricultural Research Service (USDA-ARS) Conservation and Production Research Laboratory (CPRL) in the semiarid Southern High Plains. The remote sensing (RS) campaign occurred during the 2007 summer cropping season. Daily ET estimations were evaluated using measured ET data from five monolithic weighing lysimeters located in the CPRL. On average, errors in estimating hourly ET were -0.7±11.6%; for daily ET, errors were 2.4±9.3%. Results indicated that METRIC algorithm estimated ET values well when the surface roughness for momentum transfer considered heterogeneous surface conditions and when the grass reference ET fraction was used to extra...

Groundwater recharge and chemical evolution in the southern High Plains of Texas, USA

The unconfined High Plains (Ogallala) aquifer is the largest aquifer in the USA and the primary water supply for the semiarid southern High Plains of Texas and New Mexico. Analyses of water and soils northeast of Amarillo, Texas, together with data from other regional studies, indicate that processes during recharge control the composition of unconfined groundwater in the northern half of the southern High Plains. Solute and isotopic data are consistent with a sequence of episodic precipitation, concentration of solutes in upland soils by evapotranspiration, runoff, and infiltration beneath playas and ditches (modified locally by return flow of wastewater and irrigation tailwater). Plausible reactions during recharge include oxidation of organic matter, dissolution and exsolution of CO2, dissolution of CaCO3, silicate weathering, and cation exchange. Si and 14C data suggest leakage from perched aquifers to the High Plains aquifer. Plausible mass-balance models for the High Plains aquifer include scenarios of flow with leakage but not reactions, flow with reactions but not leakage, and flow with neither reactions nor leakage. Mechanisms of recharge and chemical evolution delineated in this study agree with those noted for other aquifers in the south-central and southwestern USA.

Nitrate reduction during ground-water recharge, Southern High Plains, Texas

In arid and semi-arid environments, artificial recharge or reuse of wastewater may be desirable for water conservation, but NO 3 − contamination of underlying aquifers can result. On the semi-arid Southern High Plains (USA), industrial wastewater, sewage, and feedlot runoff have been retained in dozens of playas, depressions that focus recharge to the regionally important High Plains (Ogallala) aquifer. Analyses of ground water, playa-basin core extracts, and soil gas in an 860-km 2 area of Texas suggest that reduction during recharge limits NO 3 − loading to ground water. Tritium and Cl − concentrations in ground water corroborate prior findings of focused recharge through playas and ditches. Typical δ 15 N values in ground water (>12.5‰) and correlations between δ 15 N and ln C NO − 3–N suggest denitrification, but O 2 concentrations ≥3.24 mg l −1 indicate that NO 3 − reduction in ground water is unlikely. The presence of denitrifying and NO 3 −-respiring bacteria in cores, typical soil–gas δ 15 N values <0‰, and decreases in NO 3 −–N/Cl − and SO 4 2−/Cl − ratios with depth in cores suggest that reduction occurs in the upper vadose zone beneath playas. Reduction may occur beneath flooded playas or within anaerobic microsites beneath dry playas. However, NO 3 −–N concentrations in ground water can still exceed drinking-water standards, as observed in the vicinity of one playa that received wastewater. Therefore, continued ground-water monitoring in the vicinity of other such basins is warranted.

Cropping and Tillage Systems for Dryland Grain Production in the Southern High Plains

AbstractLow precipitation and high evaporative potential limit yields of dryland crops in the semiarid Southern High Plains. Improved residue management can reduce evaporation and improve water conservation. We compared no‐tillage (NT) and stubble mulch (SM) residue management effects from 1984 to 1993 on leveled minibenches at Bushland, TX, using winter wheat (Triticum aestivum L.)‐fallow (WF), continuous wheat (CW), wheat‐sorghum [Sorghum bicolor (L.) Moench]‐fallow (WSF), and continuous sorghum (CS) systems. The soil was a Pullman clay loam (fine, mixed, thermic Torrertic Paleustoll). Our objective was to quantify and compare soil water storage, crop water use, and grain production in order to identify the most water‐efficient production system. Relative to SM management, NT management of wheat residues increased average soil water contents at planting of the next crop by 22 mm with WSF, 15 mm with WF, and 29 mm with CW; it was not as effective with sorghum residues. Mean grain yields were not affected by residue management on any cropping system, because the additional water stored with NT management was slight in relation to seasonal evapotranspiration. Cropping systems had major effects on grain yield and production. Fallow systems (WSF, WF) normally resulted in higher yields than the corresponding annual cropping system (CW, CS). However, when grain production was adjusted to an annual basis including fallow time, the CS system was most efficient at using precipitation, producing 92% more grain than WSF, 240% more than CW, and 320% more than WF. Grain production was more than twice as great with sorghum than with wheat, due to greater biomass production and a 33% greater harvest index. Although wheat is the major dryland crop in the Southern High Plains, sorghum seems much better adapted to the region's predominant pattern of late spring‐summer rainfall.

SEASONAL AND MAXIMUM DAILY EVAPOTRANSPIRATION OF IRRIGATED WINTER WHEAT, SORGHUM, AND CORN — SOUTHERN HIGH PLAINS

Evapotranspiration (ET) is basic information required for irrigation scheduling and for crop growthsimulation models. However, many ET models have not been tested for their applicability to the Southern High Plains. Inthis study, ET was measured for irrigated winter wheat (Triticum aestivum L.), sorghum [Sorghum bicolor (L.) Moench],and corn (Zea mays L.) at Bushland, Texas, in the semi-arid Southern High Plains for various growing seasons from 1988through 1993. Weighing lysimeters containing Pullman clay loam (Torrertic Paleustolls) monoliths were used to measureET. Weather data from a nearby station were used to compute daily ET values for several widely used reference orpotential ET equations. These computed values were then compared by linear regression with the measured ET values forperiods of full groundcover (LAI=3) and with adequate soil water to permit maximum ET. Measured mean seasonal ETwas 877 mm for winter wheat, 771 mm for corn, and 578 mm for sorghum. Maximum daily ET rates rarely exceeded10 mm d1 for the sorghum or corn crops, except for a few days during a brief period of strong advection in 1990 whencorn ET rates exceeded 12 mm d1. Maximum daily ET for wheat exceeded 10 mm d1 on many days during the threeseasons due to the high vapor pressure deficits and wind speeds at Bushland during the spring and early summer. ThePenman-Monteith equation performed consistently better than other combination and/or radiation/temperature based ETequations in estimating maximum daily ET rates for these crops. The leaf diffusion resistance (rl ) permitting the bestagreement between predicted and lysimetrically determined ET was 280 s m1 for sorghum, 252 s m1 for corn, and135 s m1 for wheat when using the relationship of rc = rl /(0.5 LAI) where LAI is the leaf area index and rc is canopyresistance in s m1. These results indicate that the greater seasonal water use by irrigated corn compared with sorghum inthis environment was due mainly to the differences in planting date and growing season length since the apparent leafresistances were similar. The even higher seasonal and maximum daily water use of irrigated winter wheat compared withcorn and sorghum was due to its longer growing season, its lower leaf resistance, and the high evaporative demand in thespring in the Southern High Plains.

Chemical and Isotopic Methods for Quantifying Ground‐Water Recharge in a Regional, Semiarid Environment

AbstractThe High Plains aquifer underlying the semiarid Southern High Plains of Texas and New Mexico, USA was used to illustrate solute and isotopic methods for evaluating recharge fluxes, runoff, and spatial and temporal distribution of recharge. The chloride mass‐balance method can provide, under certain conditions, a time‐integrated technique for evaluation of recharge flux to regional aquifers that is independent of physical parameters. Applying this method to the High Plains aquifer of the Southern High Plains suggests that recharge flux is approximately 2% of precipitation, or approximately 11 ± 2 mm/y, consistent with previous estimates based on a variety of physically based measurements. The method is useful because long‐term average precipitation and chloride concentrations in rain and ground water have less uncertainty and are generally less expensive to acquire than physically based parameters commonly used in analyzing recharge. Spatial and temporal distribution of recharge was evaluated by use of δ2H, δ18O, and tritium concentrations in both ground water and the unsaturated zone. Analyses suggest that nearly half of the recharge to the Southern High Plains occurs as piston flow through playa basin floors that occupy approximately 6% of the area, and that macropore recharge may be important in the remaining recharge. Tritium and chloride concentrations in the unsaturated zone were used in a new equation developed to quantify runoff. Using this equation and data from a representative basin, runoff was found to be 24 ± 3 mm/y; that is in close agreement with values obtained from water‐balance measurements on experimental watersheds in the area. Such geochemical estimates are possible because tritium is used to calculate a recharge flux that is independent of precipitation and runoff, whereas recharge flux based on chloride concentration in the unsaturated zone is dependent upon the amount of runoff. The difference between these two estimates yields the amount of runoff to the basin.

Evapotranspiration of Irrigated Winter Wheat — Southern High Plains

Models of water use for irrigation scheduling and for crop growth simulation require validation of the evapotranspiration (ET) submodel. In this study ET was measured for irrigated winter wheat (Triticum aestivum L.) at Bushland, Texas, in the semi-arid Southern High Plains for the 1989-1990, 1991-1992, and 1992-1993 winter wheat cropping seasons using weighing lysimeters that contained undisturbed monoliths 3 3 2.3 m deep of Pullman clay loam soil (Torrertic Paleustolls). Weather data from a nearby station were used to compute daily ET values for a reference alfalfa crop (hypothetical) using the ASCE Manual No. 70 equations based on the Penman-Monteith equation and several other widely used potential or maximum ET models. Linear regressions between ET estimated from widely used equations and the reference alfalfa ET equation indicated that direct comparisons with computed ET values could not be reliably predicted with simple ratios. For the computed reference alfalfa ET base, peak basal crop coefficients (Kcb) varied from 0.88 to 1.00 for the three seasons and were lower than those reported from other locations. Peak mean crop coefficients (Kc) varied from 0.83 to 0.94 for the three seasons. Seasonal ET varied from 791 to 957 mm for the three seasons. Evapotranspiration and crop coefficients for winter wheat varied considerably with season.

Soils and landscape evolution of eolian plains: the Southern High Plains of Texas and New Mexico

Sheets of eolian sediment cover many areas of the earth's surface, sand seas, dune fields, and loess sheets being the best known examples of such features. Less well known are deposits of sandy, eolian sediment forming extensive plains. An excellent example of such a region is the semi-arid Southern High Plains (northwest Texas and eastern New Mexico). The level landscape of the area was created by deposition of multiple, extensive (≈ 80,000 km 2 ) sheets of eolian sediment (Blackwater Draw Formation) over the past 1.4+ Ma. This deposit grades from sandy (southwest) to silty and clayey (northeast) and is up to 27 m thick. Surface soils (at least 30,000 and possibly 120,000 years old) are well developed (5YR hues, agrillic horizons, 1–2m thick with prismatic structure, Stage II–III calcic horizons) and are generally Paleustolls and Paleustalfs, with some Paleargids and Haplargids. Morphologic variation is due mainly to textural variation of the eolian parent material, although locally thickness of the parent material and wind erosion and cumulization are important factors, and locally slight variation in effective precipitation may be significant. The Blackwater Draw Formation contains as many as six well-developed buried soils, each formed in individual layers of eolian sediment, similar to or more strongly expressed (2.5YR hues, higher illuvial clay content) than the regional surface soils. The presence of the buried soils indicates that sedimentation was episodic and separated by long periods of relative landscape stability. Eolian processes also appear to have been important during the periods of stability and pedoenesis by providing clayey, calcareous dust that was added to the soil, promoting formation of the argillic and calcic horizons. The sedimentologic and pedologic uniformity of the deposit suggests that the regional environment has not varied significantly during the Quaternary except for periods of increased sedimentation or wind deflation. Underlying the Blackwater Draw Formation is an Upper Tertiary deposit (up to 36 m thick) of eolian sand, silt, and clay (Ogallala Formation). This deposit contains buried soils very similar to those in the Blackwater Draw Formation, suggesting that the geomorphic processes that created the Quaternary landscape of the Southern High Plains began to operate in the late Tertiary, perhaps as much as 11 million years ago.

Playa Water Quality Changes with Time and Effects on Clarification

An estimated 2 to 5 million acre‐feet of runoff water accumulates annually in playas on the semiarid southern high plains; however, development of this large potential water source has been impeded by lack of water quality data. Most of the water is lost by evaporation, and timely utilization is essential. Therefore, a particular need exists for quality data on freshly impounded water. Water from five playas was sampled and analyzed at intervals for 24 days following runoff from a single storm. Chemical analyses showed playa waters were excellent for irrigation; however, adequate economical clarification is probably needed for such projects as direct aquifer injection. With aging, less synthetic cationic polyelectrolyte and more alum was required for water clarification. Initially low nitrate and chemical oxygen demand values decreased with time.