Meteorologic and isotopic characteristics of precipitation events with implications for groundwater recharge, Southern High Plains

Atmospheric Research, 23 (1989) 51-82

51

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Meteorologic and Isotopic Characteristics of Precipitation Events with Implications for GroundWater Recharge, Southern High Plains* RONIT NATIV .1 and ROBERT RIGGIO

Bureau of Economic Geology, The University o[ Texas at Austin, Austin, Texas 78712 (U.S.A.) Texas Water Commission, Austin, Texas (U.S.A.) (Received January 27, 1988; accepted after revision November 17, 1988)

ABSTRACT Nativ, R. and Riggio, R., 1989. Meteorologic and isotopic characteristics of precipitation events with implications for ground-water recharge, Southern High Plains. Atmos. Sci., 23: 51-82. Recharge estimates for the Ogallala aquifer in the Southern High Plains vary by two orders of magnitude (0.01 to 1.6 inches/yr or 0.25 to 41 mm/yr) because of differing interpretations of the contribution of the area's numerous playa lakes to the recharge of the aquifer. Higher rates are suggested by studies in which the playa lakes are assumed to be a major source of recharge. To reevaluate the recharge potential of precipitation, we measured meteorologic and isotopic properties of single precipitation events at five stations for one year. Values of precipitation for J~80 (-22.7%o to -4.7%0) and JD (-162%o to +35%0) plot along the world meteoric water line. Mean annual weighted values of JlsO and O~Dare -7.5%o and -48%o, respectively. Air masses from the Gulf of Mexico account for most summer precipitation, which is more isotopically enriched than winter precipitation, mainly derived from air masses from the eastern Pacific Ocean. Precipitation is more depleted in J 180 and o~Dtoward the northwestern part of the Southern High Plains, becoming isotopically enriched toward the southeast. These isotopic differences reflect temperature, continental, and altitude effects. Isotopic composition of Ogallala ground water has a spatial distribution similar to that of precipitation. Ground water, however, is consistently more enriched than precipitation sampled in the same area, suggesting evaporation during recharge. Evaporation can occur either in playa lakes prior to infiltration or in the vadose zone. Tritium in precipitation ranges from 1.1 to 14.7 TU, and in groundwater it ranges from zero where the vadose zone is thick, to 73 TU, where the zone is thin. The calculated recharge rate (using tritium as a tracer) ranges from 0.5 to 3.24 inches/yr, or 12.7 to 82 mm/yr. Such high recharge rates were only measured below playa lakes. Because of the fast recharge rates and the slightly enriched values of 180 and 5D in ground water, the Ogallala aquifer is most likely recharged by focused percolation of partly evaporated playa lake water, rather than by slow regional diffusive percolation of precipitation. *Publication authorized by the Director, Bureau of Economic Geology, The University of Texas at Austin. *1Current address: The Seagram Center for Soil and Water Sciences, Hebrew University of Jerusalem, Rehovot 76100, Israel.

0169-8095/89/$03.50

© 1989 Elsevier Science Publishers B.V.

52 RESUME Les estimations de recharge de la nappe aquif'ere d'Ogallala dans les grandes plaines du Sud des Etats-Unis varient de deux ordres de grandeur {0,25 h 4 mm/an) scion les diff~rentes hypotheses sur la contribution des nombreux lacs de la r$gion ~tla recharge de la nappe. ~ taux lesplus dlev~s sont ddduits d'$tudes oh les lacs sont supposes ~tre la source principale de recharge. Afin de r$~valuer le potentiel de recharge des precipitations, on a mesur$ les proprigt~s m~t~orologiques et isotopiques des ~pisodes individueIs de precipitation h cinq stations pendant un an: Le graphe de ~180 ( -22,7%0 h -4,7%0) et de o"D ( - 162%o h +35%0) est conforme au standard international. Les valeurs annuelles moyennes pond~rSes de ~ 180 et de 6D sont respectivement de - 7,5%~ et -48~c. Les masses d'air en provenance du Golfe du Mexique sont h l'origine de l'essentiel des precipitations d'~t~, ces precipitations ~tant plus enrichies isotopiquement que les precipitations d'hiver gdn~rSes dans des masses d'air venant de l'est du Pacifique. Les pr$cipitations sont isotopiquement plus pauvres en ~180 et o'D vers le nord-ouest des grandes ptaines du sud, et plus riches vers le sud-est. Ces differences isotopiques refl~tent les effets de temperature, de continent et d'altitude. La composition isotopique de reau souterraine d'Ogallala prSsente une distribution spatiale semblable h celle des precipitations. Cependant, l'eau souterraine est notabtement enrichic par rapport h l'eau de precipitation ~chantillonn6e dans une m~me r~gion, ce qui sugg~re le r61e de l'~vaporation pendant la recharge. L'$vaporation peut survenir aussi bien dans les lacs avant infiltration que par la zone de vadose. Le contenu en tritium des precipitations s'~chelonne de 1,1 h 14,7 TU, alors que dans les eaux souterraines il va de 0 h 73 TU suivant que l'~paisseur de la zone de vadose est forte ou faible. Le taux de recharge calcul$ (en utilisant comme traceur le tritium) va de 12,7 h 82 mm/an. Des taux aussi ~lev~sde recharge n'ont ~t6 mesur~s qu'au-dessous de lacs. Compte tenu des taux de recharge rapide et des faibles enrichissements en fi~80 et ocDde l'eau souterraine, la nappe ~luilibr~e d'Ogallala est plutft recharg$e par percolation ponctuelle d'eau lacustre partiellement ~vapor~e, que par percolation lente rSgionale diffuse des eaux de pr$cipitation. INTRODUCTION T h i s s t u d y is p a r t o f a larger p r o g r a m c o n c e r n i n g t h e c h a r a c t e r i z a t i o n of t h e Ogallala aquifer, w h i c h u n d e r l i e s t h e S o u t h e r n H i g h P l a i n s of T e x a s a n d N e w Mexico ( N a t i v a n d S m i t h , 1987). T h e Ogallala aquifer provides the m a i n w a t e r s u p p l y for t h e H i g h P l a i n s o f T e x a s a n d N e w M e x i c o (Fig. 1 ) a n d h a s b e e n severely d e p l e t e d b y e x t e n s i v e p u m p a g e for irrigation. W a t e r q u a l i t y h a s det e r i o r a t e d as a r e s u l t of c o n t a m i n a t i o n b y a g r i c u l t u r a l c h e m i c a l s , fertilizers a n d oil field b r i n e s ( N a t i v a n d S m i t h , 1987 ). A l t h o u g h r e c h a r g e into t h e Ogallala aquifer f r o m p r e c i p i t a t i o n a n d s u r f a c e w a t e r h a s b e e n widely discussed ( B a r n e s , 1949; K l e m t , 1981; U.S. B u r e a u of R e c l a m a t i o n , 1982; K n o w l e s et al., 1984; W o o d a n d O s t e r k a m p , 1984; Stone, 1984; S t o n e a n d M c G u r k , 1985), estimates of annual recharge rates vary by more than two orders of magnitude (0.01 to 1.6 i n c h e s / y r , or 0.25 to 41 m m / y r ) . T h e m a i n r e a s o n for t h i s wide r a n g e of r e c h a r g e r a t e e s t i m a t e s is v a r y i n g p e r c e p t i o n s of t h e relative i m p o r t a n c e of t h e two m a j o r r e c h a r g e c o m p o n e n t s - p r e c i p i t a t i o n a n d s u r f a c e water. R e c h a r g e r a t e is n e c e s s a r y to c a l c u l a t e f u t u r e w a t e r level declines or delineat i o n of zones w h e r e s u r f a c e c o n t a m i n a t i o n is likely to r e a c h t h e w a t e r t a b l e in t h e n e a r future.

I0~ o

IO~=

IOl o

tO0e

\\\

r//

'\ \

\\

'\ \ ;

0

Precipitation

station

sampling

EXPLANATION Precipitation

station

\ \ \

\ \ \ \ \

l

\\\

'

\ \

I

e

I

IOOrni t

,go~m

~8 / Averag.e ~nnL~ol p~'ecipitotion (inches)

I

•

Fig. 1. Precipitation distribution in Texas (Larkin and Bomar, 1983 ) and locations of study area and rain sampling stations. Contour interval is varied for the insert map and 1 inch for the study area. West-to-east and south-to-north increase in annual precipitation is the norm in the Southern High Plains because of proximity to the Gulf of Mexico and orographic effects, respectively.

04 °

J1

54 The Southern High Plains have a poorly developed drainage system, and streams that head on the Rolling Plains are generally intermittent or have a very small perennial flow. Large parts of the Southern High Plains have scattered depressions called playas that range from a few feet to 50 ft. (1 to 15 m) or more in depth and from a few hundred feet to a mile or more (1.6 km or more) in diameter. Runoff from 89% of the Southern High Plains surface accumulates in these depressions, following heavy precipitation and forms lakes (Dvoracek and Black, 1973 ). Recharge from playa lakes into the Ogallala aquifer is a controversial topic. Some researchers (Harris et al., 1972; Knowles et al., 1984) assumed that the large amounts of water stored in these lakes evaporate, primarily because they considered the clay-rich soils in the bottoms of playas to be impermeable. Because these large quantities of water are excluded from potential recharge volumes, annual recharge estimates provided by these studies are small. Other researchers (Kier et al., 1984; Wood and Osterkamp, 1984; Stone, 1984; Stone and McGurk, 1985) suggested that playa lakes are actually a major source of recharge to the Ogallala aquifer, and, consequently, their recharge estimates are much higher. We reevaluated the relative contributions of precipitation and surface water. To evaluate the recharge potential of precipitation we measured the properties of single precipitation events (an event is defined as a set of successive precipitation days), rather than studying cumulative precipitation over periods of months or years. The relations between isotopic composition of precipitation, general wind-flow pattern and the prominent source of moisture in the region prior to and during the event were explored. To date, this field of research has been underdeveloped. Some aspects of data collection, data processing and possible modes of interpretation are presented. NETWORK AND INSTRUMENTATION Precipitation was monitored and sampled from October 1984 to November 1985 at five National Weather Service stations across the study area. Four of the stations are located in the Southern High Plains at Clovis, New Mexico, and at Amarillo, Lubbock and Midland, Texas. The fifth station is located in the Rolling Plains of Texas at Paducah {Fig. 1 ). These sampling stations were selected to provide precipitation data along north-south and east-west transects and to examine the effects of altitude, topography, source of precipitating air masses {Gulf of Mexico, Pacific Ocean) and distance from the Gulf of Mexico. Precipitation was collected on a daily basis at each station using standard rain gauges. Daily samples were analyzed for ~sO/~60 and 2H/~H ratios and occasionally for tritium. Because precipitation was collected three times a day in Amarillo, Lubbock and Midland, we assume that at these stations evaporation and resulting isotopic fractionation of precipitation stored in the collecting container were generally minimal. At the Paducah and Clovis stations,

55

precipitation was collected once a day, and during warm days the samples might have undergone some evaporation in the sampling containers, resulting in isotopically enriched water. Isotopic composition of precipitation at these stations, however, did not indicate evaporation. The oxygen and hydrogen isotopic data are expressed in terms of fi notation relative to Vienna Standard Mean Ocean Water (VSMOW). Thus: ~

Rsample -- RVSMOW RVSMOW

where R is the lS0/160 or 2H/1H isotope concentration ratio. The tritium concentration is expressed as the ratio of tritium atoms to hydrogen-1. A ratio of 3H/1H = 10 - ' s is defined as one tritium unit. The ~ 1sO and riD measurements were performed at the Coastal Science Laboratory, Austin, Texas, using a Micromass 602E mass spectrometer. Tritium measurements were performed at The Weizmann Institute of Science, Rehovot, Israel. The samples were enriched by electrolysis and counted in Johnston proportional counters inside a home-built anticoincidence counter and an extensive passive shield using an on-line IBM PC. Daily precipitation quantities and isotopic compositions at each monitoring station are presented in Nativ and Riggio (1987, their appendix). METHODOLOGY

Meteorology Upper-air synoptic patterns were studied to identify major and minor troughs and ridges; to relate surface precipitation elements to upper-air features; and to examine wind and moisture fields. Satellite and radar information was gathered to track clouds and precipitation echos as they moved into and through the Southern High Plains region. The meteorological analysis used Geostationary Operational Environmental Satellite (GOES) visible and infrared imagery, as well as 850-mb, 700-mb, 500-mb and 300-mb charts, surface maps and radar summary charts developed by the National Weather Service. Origin of the moisture fields that affected the study's precipitation events was interpreted and extrapolated from storm tracks and cloud movement and from moisture-field trajectories observed in synoptic patterns at specific levels of the atmosphere.

Isotopic composition Daily sampling and measurement of precipitation was essential to allow the identification of contributing meteorological circumstances and their isotopic finger prints. Of 251 precipitation samples that were collected at the five stations during the study period, 216 daily samples were analyzed for ~lSO and O'D, and 33 daily samples were measured for tritium. Isotopic data are pre-

56 sented in this report by day, event, month, and year. With the exception of daily isotopic data, analyses were weighted by the amount of precipitation that they represent per event, month, and year, respectively, at each station. The calculation followed the equation:

~ SixP, 6 weighted by event precipitation- '= 1

P

(1)

where: 8=8180 or aD value (S&) of the precipitation sample tbr the /-day; Pi = precipitation amount during the/-day; P = precipitation amount recorded during the event; n = number of days in the precipitation event. Monthly weighted mean values were calculated based on eq. 1 but: 6i= the weighted 6180 or 8D value (%0) of precipitation for the /-event; Pi = precipitation amount during the/-event; P = total amount of sampled and analyzed precipitation during the month. Thirty-five samples (mainly from summer 1985 ) were not analyzed for 6~sO and oq) because of budgetary constraints. Instead, one or two precipitation events were analyzed for each month during summer 1985. Furthermore, a few of the monitored precipitation events were not sampled because of human error. Therefore annual mean isotopic values reported in this study for each station include both measured and estimated values. Measured annual mean values for 8180 and 6I) are based solely on samples that were analyzed, therefore they are biased to an unknown extent toward the fall, winter, and spring events. In order to adjust the mean annual values and compensate for the summer events, we calculated monthly mean isotopic values for each station by weighting the isotopic composition measured in analyzed samples (representing only part of the monthly precipitation) by the actual monthly precipitation amount. These reconstructed monthly data were then used to estimate annual weighted mean values for 6180 and oq) for each station. Calculation followed eq. 1 but: 6i=measured weighted mean of 6180 or o~D (%o) for the/-month; P i = a m o u n t of precipitation recorded during the /-month at the station; P = total annual precipitation at the station. Because tritium was measured in precipitation that was sampled between November 1984 and March 1985, we assume these data represent fall and winter events only. PRECIPITATION

Normalfeatures The normal annual precipitation in the Southern High Plains region is primarily the result of the availability of atmospheric moisture and topography.

57

Precipitation increases from 15 inches/yr (390 m m / y r ) in the west to about 22 inches/yr (560 m m / y r ) in the east, and from nearly 13 inches/yr (330 m m / yr) in the south to about 20 inches/yr (510 m m / y r ) in the north (Larkin and Bomar, 1983 ) (Fig. 1 ). The west-to-east increase in annual precipitation is the norm in the Southern High Plains because the atmosphere over the eastern part of the region is upstream in the flow of Gulf moisture and therefore contains greater amounts of precipitable water. The south-to-north increase in annual precipitation is partly due to topographic lifting of moist air over the High Plains Caprock Escarpment that borders the Rolling Plains. The orographic effect results in a greater occurrence of precipitation along and north of the escarpment, which in turn produces greater amounts of annual precipitation at the northwestern stations located at the higher elevations. For the purpose of this study, we defined the normal annual precipitation of the Southern High Plains to be the simple average of the mean annual precipitation values calculated at each of the five stations (National Oceanic and Atmospheric Administration, 1951-1980). The calculated normal regional annual precipitation was 17.66 inches (450 mm). Approximately two-thirds of the region's total annual precipitation occurs from May through August (Fig. 2). Typically, May and June are the wettest months, averaging 2.57 and 2.70 inches (65 and 69 m m ) , respectively, whereas January and December are the driest months, averaging only 0.47 and 0.53 inch (12 and 13 mm).

o

19B 4

1985

0A 7ss5

Fig. 2. Annual distribution of rainfall during 1984/85 in the Southern High Plains, compared to normal precipitation (1951-1980). During the year of study, average precipitation measured across the region was 143 percent of normal. Above-normal precipitation was measured in 9 of the 12 months.

58

Observed features By month (Table I). During the year of study, average precipitation measured across the region was 25.29 inches (642 mm), or 143 percent of normal. This amount is greater than two standard deviations (one standard deviation is 3.62 inches, or 92 mm) above normal, indicating the study year was abnormally wet in the Southern High Plains. Above-normal precipitation was measured in 9 of the 12 months (Fig. 2). Relative to normal, June, October (1985) and November were the wettest months, measuring 143, 265 and 262 percent of normal, respectively, and October (1984), May, July and August were the driest months, measuring 33, 73, 86 and 64 percent of normal, respectively. By precipitation event (Table II). During the study period, 49 precipitation events were recorded within the study area. Duration of precipitation events ranged from 1 to 10 days, with 3.3 days being the mean duration per event; however, more than half of the precipitation events did not last more than 2 days (Fig. 3 ). Precipitation per event ranged from 0.01 to 4.47 inches (0.3 to 114 mm). Trace amounts were considered to be zero. Mean precipitation per event ranged from 0.62 inches in Midland to 0.88 inches in Paducah {15.7 to 22.1 mm, respectively); more than 66% of the events resulted in amounts of 1 inch (25.4 mm) or less (Table II). TABLE I

Monthly precipitation, Southern High Plains, 1984/85 1984

Station

Amarillo Lubbock Midland Clovis Paducah

1985

Oct. *~

Nov.

Dec.

Jan.

Feb.

Mar.

Apr.

(inch) (%)

(inch) (%)

(inch) (%)

(inch) (%)

(inch) (%)

(inch) (%)

(inch) (%)

0.43 0.26 0.57 0.75 0.36

1.09 1.87 2.31 1.37 1.99

1.00 1.18 0.83 0.81 3.44

0.99 0.38 0.84 0.31 0.44

0.77 0.27 0:59 0.08 1.68

1.49 1.t9 0.63 2.54 2.70

2.79 0.48 0.21 1.31 1.31

1.8 1.0 2.9 2.9 1.2

4.5 7.2 11.6 5.3 6.5

4.2 4.6 4.2 3.2 11.2

4.1 1.5 4.2 1.2 1.4

3.0 1.0 2.9 0.3 5.5

6.1 4.6 3.2 9.9 8.8

1985

Station

Amarillo Lubbock Midland Clovis Paducah

May

June

July

Aug.

Sept.

Oct:

(inch)

(inch) (%)

(inch) (%)

(inch) (%)

(inch) (%)

(inch) (%)

3.08 4.51 2.23 2.66 6.47

2.07 3.85 0.90 1.75 1.72

1.67 0.63 0.81 3.47 0.96

4.96 4.73 3.15 4.49 2.79

3.07 3.60 5.93 3.14 5.01

0.86 2.97 0,99 2.94 1.78

3.5 11.5 5.0 11.5 5.8

12.7 17.4 11.2 10.4 21.1

8.5 14.9 4.5 6.8 5.6

6.9 2.4 4.1 13.5 3.1

20.4 18.2 15.8 17.5 9.1

* ~Data of October 1984 include precipitation from 10/24/84 to 10/31/84 only.

Total (inch) 12,6 13.9 30.0 12.3 16.3

24.27 25.92 19.98 25.62 30,65

11.5 1.9 1.1 5.1 4.3

59

v1

I

2

I

141

161

Duration (days)

I S 1

I i0 I oA 7 8 6 4

Fig. 3. Duration of precipitation events in 1984/85 ranged from 1 to 10 days, with 3.3 days being the mean duration per event; however, more than half of the precipitation events did not last more than 2 days.

By station (Table III). No station recorded precipitation in all 49 events. Although Amarillo and Clovis recorded the highest number of precipitation events (36), Paducah recorded the highest total amount of precipitation (30.65 inches, or 779 mm) and had the greatest average amount per event {0.88 inch, or 22 mm). All stations recorded above-normal precipitation during the study period. Clovis reported the greatest amount above normal (155%), followed by Midland and Lubbock; Amarillo recorded the least amount above normal ( 127% ). The western and southwestern parts of the region received the greatest abovenormal annual precipitation in the Southern High Plains from October to November, when moisture is mostly derived from the Pacific (Table I). From this we can infer that Pacific Ocean moisture played a greater than normal role in precipitation during this study. Paducah reported the largest amount of precipitation measured during a single event {4.47 inches, or 114 mm), whereas Midland reported the greatest daily total of all the stations (3.59 inches, or 91 mm). Maximum intensity of precipitation was recorded in Lubbock during a summer event (1.77 inches/h, or 45 m m / h ) . Twenty of the 49 precipitation events were measured at all five stations (Table II ). More stations recorded simultaneous precipitation during fall events

60 TABLE II Rain amounts of individual events (inches) during 1984/85, Southern High Plains 1984

1985

Station

24-26 Oct

31 Oct1 Nov

16-18 Nov

24-26 Nov

4-5 Dec

13-21 Dec

26 Dec-3 Jan

9 Jan

11-14 Jan

Amarillo Lubbock Midland Clovis Paducah

0.38 0.26 0.57 0.65 0.36

0.05

0.47 1.01 1.01 1.07 0.63

0.03 0.02

0.11 0.22 0.39 0.05 1.29

0.10 0.02

0.06 0.07 0.44 0.11

0.16

0.88 0.95 0.45 0.76 2.15

0.03 0.04

0.10 0.01

0.62 0.86 1.31 0.30 1.35

13 May

16-25 May

31May6 June

9-14 June

16-18 June

21,22 June

25-26 June

1-5 July

2.12 3.08 1.07 2.06 4.47

0.48 0.35 1.15 0.57 0.18

0.30 0.64 0.01 0.03 0.32

0.03 0.14

0.15 0.50 0.04

T 0.56 0.49 0.77

0.16

0.85 2.39 0.95 2.94 1.62

1.26

0.21

10-16 Sept

18-22 Sept

28-29 Sept

1 Oct

8-15 Oct

17-22 Oct

2.21 0.24 1.84 1.60 0.67

1.80 3.48 0.50 1.22 0.87

0.95 1.01 0.63 0.55 1.25

0.07

3.00 2.94 2.09 2.63 2.09

0.07 0.64 3.77 0.51 2.91

1985 Station

4-7 May

Amarillo Lubbock Midland Clovis Paducah

0.01 0.35

1985 Station

Amarillo Lubbock Midland Clovis Paducah

4 Sept

0.18 1.12

but recorded the greatest number of localized precipitation events during spring a n d s u m m e r ( F i g . 4 ) . T h i s is t o b e e x p e c t e d b e c a u s e f a l l p r e c i p i t a t i o n e v e n t s are associated with large-scale synoptic features producing widespread precipitation, whereas summer events are triggered by more localized features such as afternoon thermals (a pocket of rising air due to intense solar heating). SOURCES OF PRECIPITATION

General features T h e G u l f o f M e x i c o is t h e p r o m i n e n t s o u r c e o f m o i s t u r e f o r m o s t p r e c i p i t a tion events in the Southern High Plains and Rolling Plains. Under certain

61

1985 26 J a n 4 Feb

10 Feb

20-24 Feb

28Feb1Mar

15-16 Mar

19-21 Mar

O.88 0.27 0.44 0.10 0.34

O.O3 T

0.74 0.24 0.10 0.08 1.60

0.01 0.42

0.03 0.13 0.14

0.02

0.06

1.44 0.74 0.49 2.46 1.88

20 July

22-29 July

31 July

0.13

2.02 3.29 0.28 0.49 1.32

26 Mar

29-30 Mar

11-14 Apr

16-17 Apr

0.67 0.01 0.05 0.21 0.03

T T 0.14 0.05

0.35

0.02 0.17 T 0.08 0.39

4-5 Aug

7-8 Aug

8-9 Aug

10-15 Aug

21 Aug

0.35

0.03 T 0.01 0.03

T

0.15 T

21 Apr

25-29 Apr

0.02 0.45

2.12 0.47 T 1.05 0.83

23-24 Aug

27 Aug

0.13 0.15

0.04

1985 10 July

15-18 July

0.03

0.02 T

T 0.06 0.40

T

=

0.43

0.10 0.16

0.04 0.08

1.18 0.44 0.76 1.44 T

0.47 0.45

1.20 0.35

0.15

trace

TABLE III Precipitation characteristics,Southern High Plains, 1984/1985 Station

No. of Normal Observed Percent Average Maximumevent events (inches) (inches) o f perevent normal (inches) inches date

Maximum

daily

inches date

Maximum

hourly

inches date

Amarillo Lubbock Midland Clovis

36 34 32 36

19.10 17.76 13.70 16.48

24.27 25.92 19.98 25.62

127 146 146 155

.67 .76 .62 .71

3.00 3.48 3.77

10/08-15/85 1.99 04/28/85 1.19 09/18-22/85 2.65 07/25/85 1.77 10/17-22/85 3.59 10/17/85 0.90

04/28/85 07/25/85 09/12/85

2.94

05/16-25/85

2.02

06/05/85

--

--

Paducah

35

21.86

30.65

140

.88

4.47

05/31-6/6/85

3.15

06/05/85 --

--

62

(o) t0/04-10185 20-

8

--

(c) Sprino 3105-5185 e o

.,.

m-

.][.

--

0

(b) Winter t2/84-2185

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I W2131415 Number of stotions

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v i v2 v3 v4u 5 ' Number of stotions

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o I:ii::ii::i!::iil ?! I

1 1 2 1 3 1 4

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Number of stotions

Fig. 4. Number of recording stations reporting precipitation during individual events in the Southern High Plains. (a) Annual: 10/84-10/85; (b) winter: 12/84-2/85; (c) spring: 3/85-5/85; (d) summer: 6/85-8/85; (e) fall:9/85-11/85. Twenty of the 49 precipitation events were measured at all five stations. More stations recorded simultaneous precipitation during fall events but recorded the greatest number of localized precipitation events during spring and summer.

synoptic conditions, however, the eastern Pacific Ocean provides significant amounts of moisture. The influence of the Gulf of Mexico on eastern New Mexico and Texas precipitation is evident in the distribution of annual precipitation across the region (Fig. 1 ). Precipitation decreases to the west as a result of increasing distance from the Gulf of Mexico, the major source of atmospheric moisture. The remoteness of the Southern High Plains from the Gulf increases the relative influence of other moisture sources, particularly during the winter months. Southern High Plains precipitation events acquire much of their moisture from low- and mid-level wind patterns (Scoggins et al., 1978). During the summer months the normal low,level wind pattern across the Southern High Plains and Rolling Plains typically is from the Gulf; that is, from the south-southeast.

63 (O)

(b)

N

N

//

I/

S

-//~"~..i ~ /,'

,' / /

~

~

.~"'~

~----\ \ , ,

S

\~

,\\

QA 71~t

Fig. 5. Wind roses for (a) winter and (b) summer at the Lubbock rain station (data for 19611980 from Larkin and Bomar, 1983). Numbers indicate the time (percent) of a specific wind direction during one representative month. During the summer months the normal low-level wind pattern across the Southern High Plains and Rolling Plains typically is from the Gulf; that is, from the south-southeast. In the winter, however, the normal low-level wind pattern becomes more westerly.

Pacific

Mixed

it t

"6

OI

i

=

~'~-""

o

!

l

I

I

L

I

l

I

Gulf

0

!

0

l

!

N

1984

!

DI

J

I

F

I

M

A

M

I

J

1985

I

J

!

A

!

l

S

o

oA 7~o

Fig. 6. Annual distribution of sources of precipitating air masses in the Southern High Plains, 1984/85. The Gulf of Mexico is the prominent source of moisture for most precipitation events in the Southern High Plains and Rolling Plains. However, some Southern High Plains precipitation events, particularly those that occur in the winter, acquire their moisture from a source to the southwest, most likely the southeast Pacific Ocean.

64 In the winter, however, the normal low-level wind pattern becomes more westerly (Fig. 5). This suggests that some Southern High Plains precipitation events, particularly those that occur in the winter, acquire their moisture from a source to the southwest, most likely the southeast Pacific Ocean. Examination of wind and moisture fields during precipitation events in 1984/85 confirmed these assumptions (Fig. 6). On rare occasions (once every 3 to 5 years ), a tropical storm, which may be a hurricane prior to making landfall, can contribute an abnormally large amount of rain to the Southern High Plains in a relatively short period of time (Bomar, 1983 ). However, ~ r i n g the study period, tropical storm weather spawned in the Pacific or the Gulf had no effect on the study's precipitation events.

Seasonal features No two precipitation events develop and propagate in exactly the same manner. Each precipitation event observed during the study had characteristics that were unique from the other precipitation events. However, certain synoptic-scale characteristics were found to be common amongmost of the precipitation events, which allowed us to categorize each event into generalized weather patterns. Clearly, the weather patterns observed during the study period do not represent the only weather patterns that affect the Southern High Plains. The reader is referred to Elliott (1949) for background information regarding other weather patterns that may have had an effect on precipitation events recorded in the study area. Seven general weather patterns account for most of the broad-scale precipitation events observed in the U.S. (Etliott, 1949 ). The generalized summer and winter weather patterns most frequently fit the synoptic-scale characteristics of this study's precipitation events. Although the two synoptic weather patterns are related to a particular source of moisture, they were not used as the basis for assigning a moisture source to the precipitation events. The moisture sources were assigned to each precipitation event based on detailed analyses of mesoscale, low- and mid-level wind and moisture trajectories interpolated from twice-daily synoptic charts and upper-air soundings, three-hourly surface and radar charts and hourly satellite photographs. The analyses covered the period of six hours prior to the start of the precipitation event to the end of the event. The method to determine the moisture sources for each precipitation event in the study was subjective and highly dependent on the experience and expertise of the meteorologist. Radar summary charts were first used to track the life history of each precipitation event; the observed radar characteristics were next correlated with surface- and upper-level wind and moisture patterns that were analyzed with surface, 850-mb, 700-mb and 500mb synoptic charts. Once the trigger and moisture areas associated with each precipitation event were identified, they were subjectively regressed to the

65 (a) Type Bn-a

(b) Type CH

QA 7859

Fig. 7. Seven general weather patterns account for most of the broad-scale precipitation events

observed in the U.S. (Elliot, 1949). The generalizedsummer (a) and winter (b) weatherpatterns most frequentlyfit the synoptic-scalecharacteristics of this study's precipitation events. moisture source of the precipitation event using satellite charts and upper-air synoptic charts. Summer pattern. The summer weather pattern (weather type Bn-a of E1liott, 1949) is a northwest to southeast flow pattern with an upper-level ridge in the western U.S. and a trough in the eastern U.S. (Fig. 7a). Surface low

66 pressure tends to track across the northern tier of states with a trailing cold front that often extends to the Southern High Plains before becoming stationary. Leeward troughing is evident at the surface, extending north-south through the study area (Fig. 7a). The semipermanent Bermuda High pressure cell northeast of the Floridian peninsula causes seasonal southeasterly flow, providing the region with ample amounts of moisture from the Gulf of Mexico (Elliot, 1949). A strong highpressure area, typically centered in the Great Basin region, inhibits the flow of Pacific moisture into the Southern High Plains during the summer (Elliot, 1949). Numerous rainshowers and thunderstorms are triggered in the Southern High Plains by a dry line (the boundary between a dry and a moist air mass) that moves from eastern New Mexico in advance of settling cold fronts and wedges under the moist air mass from the Gulf of Mexico. Typically, the Gulf of Mexico is the moisture source for precipitation that develops during this summer weather pattern. Winter pattern. The winter weather pattern {weather type CH of Elliott, 1949) is characterized by an upper-level pressure trough centered over the Pacific Ocean and Baja, California (Fig. 7b). The pressure surface is characterized by a large polar air mass centered over western Canada having central pressures of usually 1040 mb or greater. Typical winter precipitation events in theSouthern High Plains and Rolling Plains begin with the Baja low pressure cell advecting Pacific moisture from the southeast Pacific across northwest Mexico into the Southern High Plains region (Fig. 7b). The western Canadian dome of cold air slides rapidly southward through the Great Plains region and across Texas. Pacific moisture at the low- and mid-levels of the atmosphere overruns the dome of cold Canadian air that has settled over the Southern High Plains region and precipitates rain, sleet and/or snow, depending on the thickness of the cold air mass. The thicker the cold air mass, the greater the depth of cold air the precipitation will fall through before reaching the ground, and therefore the better the chance the precipitation will turn into snow. The winter weather pattern typically produces precipitation derived mainly from Pacific moisture or from a mixture of Pacific and Gulf of Mexico moisture.

Summary The Gulf of Mexico was the principal moisture source of most of the 49 precipitation events measured (Fig. 6). Nineteen events acquired precipitable moisture from the Gulf, 13 from the Pacific, and 17 were mixed or undetermined. Precipitation of Gulf of Mexico moisture occurred primarily in spring and early summer, whereas fall and winter precipitation was derived from Pacific moisture (Fig. 6). During late summer (August and September ), moisture sources for the Southern High Plains varied between Gulf and Pacific sources.

67 From October to early March, Pacific moisture was the primary source of precipitable moisture because the Baja low dominated the regional weather pattern. In late March the Gulf of Mexico became the dominant source of moisture and remained so through July. ISOTOPIC COMPOSITIONOF PRECIPITATION General features

Isotopic composition of precipitation in the Southern High Plains varies widely {Table IV). Values for ~180 ranged between - 22.7 and + 4.9%c and for 8D from - 1 6 2 to + 35%0, with the mean annual weighted values for all five stations being - 7 . 5 and -48%0, respectively. Deviations from the mean at each station were by factors of 2 to 3 (Table IV). Weighted annual mean values for 8180 at the Amarillo and Clovis stations ( - 8 . 9 and -8.8%0 ) are lighter than values measured in the Lubbock, Midland and Paducah stations ( - 7.3, - 6 . 3 and -6.2%o, respectively). Rainwater was also sampled in central and eastern New Mexico by Hoy and Gross {1982), Yapp {1985) and Chapman (1986). Isotopic values of rainwater in the Southern High Plains are similar to values observed in eastern New Mexico. Values for 5180 range from - 18.7 to - 0 . 2 %o with a weighted annual mean of -6.0%0 in the Roswell Basin (Hoy and Gross, 1982), and from -22.5 to -3.1%o in the Delaware Basin {Chapman, 1986). Values for gD ranged from - 162 to - 13%o in the Delaware Basin {Chapman, 1986) and from - 1 5 8 to -6%0 with a weighted annual mean of -60%o in central New Mexico (Yapp, 1985). Differences in the reported ranges in this study and studies in New Mexico probably result from different sampling methods. Extreme values are not readily masked by daily samples collected in this study as opposed to monthly or bimonthly samples in studies by Hoy and Gross (1982) and Yapp (1985). Values of 8180 and 6D fall near the world meteoric water line (Craig, 1961 ) (Fig. 8). However, linear regression between ~180 and gD yielded a correlation coefficient of r= 0.97 with sample size of 216 and a best-fit line equation of: o¢D=6.8 ~180+0.1 rather than along Craig's line with the equation: 5D =8.0 ~180+ 10 Hoy and Gross ( 1982 ) also calculated smaller slope and intercept for eastern New Mexico data, where precipitation was collected on a monthly or bimonthly basis in the Roswell Basin. Their regression line equation was: gD = 7.27 ~180+5.36 Dansgaard ( 1964 ) pointed out that worldwide sampling stations where rain-

Oo

T A B L E IV Isotopic composition of precipitation during 1984-1985 ranges of daily samples a n d weighted annual means Station

Annual rain

Annual ~ ~80 (%o)

Annual gD (%o)

reported

analyzed

range

estimated mean

measured mean

range

estimated mean

measured mean

range

measured mean

Amarillo Lubbock Midland Clovis Paducah

24.27 25.92 19.98 25.62 30.65

15.1 22.22 13.09 21.06 21.91

-18.2 -+4.9 -18.2-+3.4 -22.7 -+2.7 -21.8 -+1.6 -16.1 -+1.2

-8.9 -7.3 -6.3 -8.8 -6.2

-8.6 -6.6 -6.3 -9.1 -6.8

-134 -+14 -126-+22 -162 -+35 -155 --7 -110--1

-54 -48 -39 -62 -37

-60 -48 -40 -65 -39

1.1-14.3 4.0- 9.3 6.2-14.7 2.4-12.0 4.0-10.8

7.6 7.6 9.5 7.4 7.0

Southern High Plains

25.29

-22.7 -+4.9

-7.5

-7.5

-162 -+35

-48

-50

1.1-14.7

8.04

*~Tritium measurements were t a k e n at all stations between 11/84 and 3/85.

Annual tritium*

69

+3o- (o)

+30-

o~"

8D =88 is

-I0

8D:88 f8 0.10

"" -I0

/

-5C

-50

o (X3-9C

-90

-130 / .

./

-170

, , , , , , , , , , I ,

-23

i

/ /"

Range of ground water values in the Southern High Plains as shown in graph below

o/

-130

-7

-

( C)

~D=8~ l e O + l O y/

-

.~ ~-40_ _ 6 ° / :~W ' ~

-170 -23

+I

-15~ 180 (%o1 20

•

~

,

'

,

I

|

~

i

I

I

!

i

-7 - 1 5 ~ 1 8 0 (%0)

I

I

!

"1

+1

+

-

,-¢/ I~<'..."~'+ &~, ~ . ~ ' ~ _ 13,BmJ ~" ~;L ~ ~d~ ~ /0~ ~ f 6R " -'

,/ %

EXPLANATION o Ogollalo (north) a Ogallala (south] • Cretaceous 0 Triossic A Permian X Playa water -~..Playa well , + hne,loke)So

...

OA 7858

818 0 (°/=.,)

Fig. 8. 61sO versus 6D for (a) 216 unweighted daily precipitation samples, (b) weighted precipitation samples (by event precipitation) and (c) ground water from various stratigraphic units in the Southern High Plains. Precipitation values of 61s0 and 6D fall near the world meteoric water line, however, linear regression between 6 ~sO and o~ yielded a line equation of o~) = 6.861sO + 0.1, reflecting a temperature effect. Isotope composition values of ground water in the Ogallala and underlying aquifers scatter along the meteoric line and become concentrated on the heavy side.

water falls along a meteoric line with a slope smaller than 8 are characterized by dry or periodically dry climate. He suggested that during nonequilibrium evaporation of falling raindrops, the kinetic effect on deuterium is much smaller than that on oxygen-18, resulting in a smaller slope. This explanation can apply to both New Mexico and the Southern High Plains. When the isotopic data of winter, Pacific-originated precipitation of the Southern High Plains are plotted separately, the regression equation is: #D=7.1 61sO+5.01 whereas the isotopic data of summer, Gulf-originated precipitation plot along the line:

70 0"D= 6.8 3180 - 1.90 The differences in slopes and intercepts reflect a temperature effect, which will be discussed later. Tritium in precipitation collected in the Southern High Plains during fall 1984 and winter 1985 ranged from 1.1 to 14.7 TU (Table IV). The weighted mean of tritium concentration for all five sampling stations is 8 TU. The natural tritium abundance in precipitation (about 5 TU ) was completely masked by tritium produced by atmospheric hydrogen-bomb tests that began in 1952 in the Northern Hemisphere, reaching a peak of 10,000 TU in a single monthly rain sample in the U.S. in 1963 (International Atomic Energy Agency, 1969). The observed low tritium concentration in Southern High Plains precipitation is approaching the natural level of tritium concentration in precipitation and may indicate near-complete scavenging of anthropogenic tritium. Similar low values were also observed by Nativ and Mazor (1987) in the Negev Desert, Israel. It is interesting to note that winter precipitation typically has lower tritium concentration, whereas summer rainwater has higher concentrations (Persson, 1974). Because only fall and winter precipitation samples were analyzed for tritium during this study, it is possible that higher concentrations of tritium are present in summer precipitation in the study area.

Controlling effects on isotopic composition o/precipitation The ~lsO and 513 composition of precipitation is controlled by a variety of parameters such as temperature, altitude and condensation history of the contributing air mass (Dansgaard, 1964). The isotopic composition of precipitation depends on the temperature at which seawater is evaporated into the air and, even more importantly, the temperature of condensation at which precipitation is formed (Dansgaard, 1964 ). Seasonal variations in temperature cause enrichment of the heavy isotopes in precipitation during the summer because of elevated temperatures and cause depletion of the heavy isotopes during the cold winter. Dansgaard (1964) also suggested that during extensive precipitation events, precipitation becomes more depleted in heavy isotopes; he assumed that reduced evaporation during these events accounts for this mechanism. The history of the precipitating air mass also controls t h e composition of precipitation. As the air mass travels inland from the ocean and later ascends higher terrain, moisture gradually condenses. Because condensation of isotopically enriched (heavier) water molecules is more efficient, the isotopic composition of the air mass becomes progressively lighter as it moves inland (continental effect) and reaches higher elevations (altitude effect). The composition of a single precipitation sample reflects each of these effects. All of these parameters affect the isotopic composition of precipitation in the Southern High Plains. Precipitation is less depleted during the summer

71

(Fig. 9 ), primarily as a result of higher temperatures and increasing evaporation. Maximum values of 61sO and o~D, which amount to ÷4.9 and ÷35%0, respectively (Fig. 8), may indicate evaporation during rainfall. However, because the rainy season in the study area is late spring and summer and is characterized by numerous convective thunderstorms and rainshowers, the increased amount of precipitation may cause isotopic depletion and offset the influence of higher temperatures. It is clear, however, that the amount of precipitation has only a minor effect on the composition of precipitation compared with the combined influence of temperature and evaporation (Fig. 9). Winter precipitation is depleted by as much as - 2 2 . 7 and -162%o in 61sO and 6D, respectively, as the result of colder temperatures and reduced evaporation. Different origins of precipitating air masses for spring and summer events versus winter events also contribute to isotopically distinctive precipitation. The long distance and high elevation that air masses coming from the .

(a)

(b)

(c) - 4D

+I-

~

3.0~ c -7

8

O

2.0 .& -15

k -23

O=NI D I J = F I M ~ A=M=J ; J ' A I S

IO=NIDIJ=F=M=AIM'j'jIAI

1.0

S

=OIN=DIJ=FIMIA=M=jIj~A Monlhs

~

0

(e)

(d)

•4.0

+1-

3.o

8

O (X)

2.0 ~. -& Gw 1.0

-2~ ' I o I N I D I j I F I M I A I M I j I j Months

S

OINIol jIFIMIAIMI Months

JI JI A I S

0

OA 7851

Fig. 9. 6~sO in precipitation (bold line) and amount of rainfall (thin line) variations with time during 1984/85: (a) Amarillo, (b) Lubbock, (c) Midland, (d) Clovis, (e) Paducah. Precipitation is less depleted during the summer, primarily as a result of higher temperatures and increasing evaporation. The amount of precipitation has only a minor effecton the composition of precipitation compared with the combined influence of temperature and evaporation.

72

Pacific cross in order to reach the study area produce an isotopically depleted precipitation in winter as a result of both continental and altitude effects (Fig. 10). Lower altitudes and a shorter travel distance from the Gulf of Mexico characterize the pathway of all contributing air masses during summer and result in less depleted precipitation during this season. Observations from New Mexico indicated similar patterns of enriched precipitation during summer and more depleted precipitation during winter (Hoy and Gross, 1982, Appendix A3; Yapp, 1985, fig. 3). Because condensation-temperature, continental and altitude effects enrich isotopic composition of precipitation in the summer and deplete it in the winter, it is not clear which of these effects is predominant. Isotopic composition of precipitation varies within the study area. As was mentioned earlier, precipitation in Clovis and Amarillo is more depleted than

+ 30 ),<

X -IO

+

X +

×

...i.+ X

o

X °

-50 ¸

c-', (.,'0 -90 9. X EXPLANATION d~

-130

o Pacific +

Mixed

X Gulf

-170

i

-23

,

!

l

i

i

-15

I

-7

~le 0 (%o)

,

,

I

I

i

+I

oA z857

Fig. i0. J1"O and JD (weighted by event) in precipitation varied with the source of the precipitating air mass during 1984/85. Air masses coming from the Pacific during the winter produced an isotopically depleted precipitation as a result of both continental and altitude effects. During the summer, when allcontributing air masses came from the Gulf of Mexico, these effectsresulted in less depleted precipitation.

73

in Lubbock, Midland and Paducah (Table IV). Because stations in Clovis and Amarillo are located at higher elevations (4250 and 3590 ft. or 1300 and 1090 m above sea level) than those in Lubbock, Midland and Paducah (3200, 2800 and 1850 ft. or 975, 850 and 560 m above sea level, respectively), we assume that spatial distribution of isotopic composition of precipitation in the study area is controlled by a local altitude effect (Fig. 11 ). Continental effect also could account for the spatial distribution of isotopic composition of precipitation. During the spring and summer, most of the precipitating air masses come from the Gulf of Mexico. Spring and summer rainfall at Midland, Lubbock or Paducah is therefore expected to be less depleted than precipitation at the more remote stations of Amarillo and Clovis (Fig. 12). However, during the winter, when most of the contributing air masses come from the eastern Pacific, precipitation is expected to be more depleted at the eastern stations (Paducah and Lubbock) than at the western station (Clovis) because of the

+30X

o*X

-I0-

o

A

~ : ~ +~ ° X "

-50-

(:3 60

_ o

-90@

- 23

EXPLANATION o Amarillo + Lubbock X Midland • Clovis Z~ Poducoh

"~ °e

-150--

-170

•

i

1

I

|

I

i

I

-15

I

-7

~180 (%0)

I

I

I

I

I

+1

OA -,,e~

Fig. 11. ~ ' " 0 and 5D (weighted b y event) in p r e c i p i t a t i o n varied w i t h al t i t ude o f the sampling

stations during 1984/85. Precipitation in Clovis and Amarillo which are located at high elevations is more depleted than in the lower Lubbock, Midland and Paducah stations.

74

Gulf _5. N

5J~

s

w

E

Pocific S W

N

E

5122/85 3/29- 3/30185

-to/85

-15-

H/24-11/25/84

-2o-

-25, A

i L

i M

i

i

i

L

M

i L

! P QA 7853

Fig. 12. Spatial variation of 5~sO in precipitation samples during precipitation events associated with air masses originating from the Gulf of Mexico and the eastern Pacific Ocean. Station codes: A=Amarillo, Texas; L=Lubbock, Texas; M=Midland, Texas; C--Clovis, New Mexico; P = Paducah, Texas. Continental effect can account for the spatial distribution of isotopic composition of precipitation.

continental effect. Fig. 12 shows that indeed during some of the winter events, the Lubbock station receives more depleted precipitation than does the Clovis station. However, Paducah, which is farthest east, receives precipitation that is similar or even less depleted than that at Clovis. The reason for this observation is unknown. For the present study, emphasis was placed on the sampling of daily precipitation fractions instead of the more common method of sampling monthly or bimonthly cumulative precipitation. Daily precipitation samples from 21 larger events, those that lasted two or more days, were analyzed. The data from Amarillo, Lubbock, Midland, Clovis and Paducah stations constitute 40 sets, which are shown in Fig. 13. In 26 of the 40 sets, precipitation sampled during the first day was more enriched in the heavier isotopes than precipitation in subsequent days, which reflects a front effect. Ten of the sets showed enrichment of the heavier isotopes as the event proceeded, whereas four sets showed no significant pattern. Isotopic enrichment occurred during 10 precipitation events. Two of these events (December 13-16, 1984 and April 26-29, 1985) were studied in a more detailed fashion using twice-daily upper-air charts, three-hourly surface and radar charts and a network of twice-daily atmospheric wind and moisture vertical profiles measured at 15 upper-air stations located throughout Mexico and the southwestern U.S., including Texas. Pockets of moist air,:identified as having dew point temperature depressions of 5 ° C or less, were tracked from their moisture source to the Southern High Plains region every 12 h during the period of the precipitation events using available wind data. Because the lower

75

/"

/

.

'I

/I

o

(,o

l

-IOO

-2OO

i i

""

\

\ i

/

j t

-IO O co (X)

VI

\ I

/

-2C

E~nl No

60

Amarillo

fO

.....

Lubbock

2f 24 EXPLANATION

250

25b

......

- -

Clovis

Midland

34

44

........

';5

Paducah OA 7854

Fig. 13. Isotopic composition of precipitation in the first and subsequent days of individual events during 1984/85. The data from Amarillo, Lubbock, Midland, Clovis and Paducah stations constitute 40 sets. In 26 of the 40 sets, precipitation sampled during the first day was more enriched in the heavier isotopes than precipitation in subsequent days, which reflects a front effect.

level of the atmosphere provides the greatest amount of moisture to make precipitation (Scoggins et al., 1978), the wind analyses were confined primarily below the 10,000-ft. (3050-m) level. It appears that in both events, Pacific moisture was evident in the study area at the beginning of the event (December 13, 1984 and April 26, 1985 ) and produced precipitation depleted in heavy isotopes. As the event proceeded (December 14-15, 1984 and April 27-29, 1985) a transition zone followed; the Gulf became the dominant source for moisture and produced precipitation less depleted in heavy isotopes. Later on, precipitation eventually became lighter again, reflecting amount effect (Dansgaard,

76 1964). Based on this detailed study of two events we suggest that the enrichment of heavy isotopes in precipitation during some events is the result of the arrival of a new precipitating air mass. Eleven of the 40 sets were also analyzed for chloride content. Chloride concentration generally changed directly with isotopic composition. Chloride decreased after the first day of precipitation in conjunction with the observed depletion in isotope composition and increased with enrichment of the isotopic composition. Therefore, front effect also can be recognized in the chloride composition of precipitation in the study area. Tritium concentrations in precipitation exhibit no recognizable pattern of spatial distribution. Because spring and summer samples were not measured for tritium, variations with time and seasons that have been observed elsewhere (Persson, 1974) could not be traced. GROUND-WATERRECHARGEBY PRECIPITATION Identifying the timing of ground-water recharge by precipitation is important because it may indicate the amount of water available for ground-water recharge. Precipitation events were recorded at each station every month during the year of study. However, at each station more than 60% of the annual precipitation occurred within five months during the summer and fall {May to July and September and October) (Table I ). During these seasons, temperature and therefore evaporation are high, and rain intensity is much greater as a result of the convective nature of precipitation events. Also, water consumption by crops is at its peak. Therefore it can be argued that large amounts of precipitation are not available for ground-water recharge, as most of the water is lost to evapotranspiration, runoff and surface-water evaporation. Conversely, during winter, the temperature is lower and evaporation is reduced {although low ambient humidity causes some sublimation of snow and ice); vegetation cover and, therefore, water consumption are significantly lower, and intensity of precipitation is diminished. Also, some of the winter precipitation forms snow, which melts slowly. All these winter characteristics result in a better chance for ground-water recharge. In contrast, during very cold weather frozen-ground conditions may inhibit local recharge temporarily and heighten net sublimation losses because daytime melting of snow and ice would not produce recharge. Identification of timing of ground-water recharge can be facilitated by the use of stable isotopes. Because precipitation in winter is isotopically distinct from that in spring and summer, comparison of 5~sO and oCD compositions of precipitation and ground water may indicate the timing of most recharge occurrence throughout the year (assuming no other existing processes that may isotopically affect the percolating water). Fig. 14 presents the distribution of 5~80 in ground water in the Ogallala aquifer in the Southern High Plains. To facilitate the comparison of isotopic

77

104°

.oor~,

r

I~+'' ' i to--r-

'

I

---x

IOf e I Ro~lrt|

102°

103o

J

JHgtchlnlon

I

"t

dhom

I

/ .... J

100= Hemphlll

I

~=

GROUND WATER SAMPLES J 0 From the Ogallala aquifer

o-Z.3

: -l+.g

EXPLANATION

°6.z

• i~__~ linQsworth

From the aquifers in the Quaternary deposits

/

|-71 ~180 value of ground .4 water sample {%=} rlu~= ,,~]Line of equal ~180 'C~ld / in ground water in lhe ogallolo aquifer !I

-z.b "~t~

p...,_]

I°'+r',,c'°v+si "-.-.,,

l

--

I

J L

1%+

|.;31 Precipitation sampling station and its ~180 weightedannual mean

J

_-Paducah

54o

6.3

=

-4.2 I~

33°

N

I

I-

I

i i

I N MEX TEXAS

3Zo !

,I

I

0 I

0

i

i

IOOmi i 150kin

OA 7855

Fig. 14. Spatial distribution of 5180 in ground water in the Ogallala aquifer in the Southern High Plains and in Quaternary deposits in the Rolling Plains. Ogallala ground water is more depleted in 5180 toward the northwest and becomes isotopicallyenriched toward the southeast. Ground water in the Ogallala aquifer becomes locallymore depleted along the eastern escarpment. In Floyd County, upwelling of typically depleted ground water from the Triassic D o c k u m Group into the Ogallala aquifer was indicated. Spatial distribution of ~ IsO and b'D in precipitationalso indicates depletion in these isotopes toward the northwestern part of the Southern High Plains, which is related to altitude and continental effects.However, ground water is generally slightlymore enriched in any location than precipitation sampled in the same area.

78 composition in precipitation sampled at Paducah with the local ground water, data from ground water in the Quaternary deposits in the Rolling Plains were also included. Their composition variations were discussed elsewhere (Nativ and Smith, 1987). Ground water in the Ogallala aquifer is more depleted in 61sO toward the northwest and becomes isotopically enriched toward the southeast. The same pattern was observed for D distribution. Ground water in the Ogallala aquifer becomes locally more depleted along the eastern escarpment in Briscoe, Floyd and Crosby Counties. In Floyd County upwelling of ground water from the Triassic Dockum Group into the Ogallala aquifer was indicated (Nativ and Smith, 1987). Dockum Group water is typically more depleted than Ogallala water (Nativ and Smith, 1987; Dutton and Simpkins, 1986) and can account for the local depletion in the Ogallala ground water in this area. However, reasons for isotopic depletion in ground water in Briscoe and Crosby Counties are unknown and should be further explored. Spatial distribution of 61sO and 5D in precipitation also indicates depletion in these isotopes toward the northwestern part of the Southern High Plains, which is related to altitude and continental effects. However, ground water is generally slightly more enriched in any location than precipitation sampled in the same area (also Fig. 8c). For example, in the northwestern part of the study area 61sO generally varies in ground water from - 8.3 to - 7.1%~, whereas weighted annual mean values of 61sO in precipitation in Clovis and Amarillo are - 8 . 8 and -8.9%~, respectively (Fig. 14, Table IV). Similar trends were observed in other areas (Fig. 14). As was mentioned earlier, the annual mean was calculated by using the weighted isotopic values of each precipitation event, and therefore the isotopically enriched events of the summer are well represented because of the larger volume of precipitation during summer events. Despite use of this technique, ground water is still slightly more isotopically enriched than the annual weighted means calculated for the local precipitation sampling stations. Because the study period was significantly wetter than normal, especially during early winter when Pacific (isotopically depleted) moisture is prevalent, the possibility that precipitation sampled during this one year was more depleted than usual cannot be eliminated. Perhaps during "normal" years, precipitation is isotopically more enriched and resembles the local ground water. Based on these observations we assume that recharge certainly takes place during summertime, despite the increased surface runoff and evapotranspiration. (An even larger difference between precipitation and ground-water composition would be expected if most of the recharge occurred during the winter. ) Possible explanations for isotopic enrichment of ground water relative to precipitation are (1) aquifer recharge from playa lakes in which evaporation of precipitation and surface runoff water results in enriched isotopic composition of the residual water (Craig and Gordon, 1965; Zimmermann et al., 1967; Zimmermann, 1979 ) and (2) evaporation of percolating vadose water close to

79 land surface. Isotopic compositions of ground water plot along the meteoric water line (Fig. 8c) and become concentrated on its heavy side. However, it was found that precipitation samples that fall within the range of playa-lake and ground-water samples form 60% of the total precipitation sampled. Only 21% of the sampled precipitation had lighter isotopic composition than playalake and ground water. This observation and the absence of any shift of playalake and ground water away from the meteoric line suggest that ( 1 ) evaporation of precipitation in playa lakes prior to infiltration is small, and (2) diffusive percolation resulting in isotopically enriched water is not a dominant recharge mechanism. Tritium concentration of ground water in the Ogallala aquifer in the Southern High Plains ranged from 0 to 73 T U (Nativ and Smith, 1987). High tritium concentrations were measured in ground water in the southern part of the Southern High Plains, where the unsaturated zone overlying the aquifer is relatively thin, suggesting that the aquifer there is recharged rapidly by modern water. Tritium concentration in ground water in the rest of the area is essentially zero, apparently because of the thicker vadose zone (Nativ, 1988, fig. 21 ). The tritiated precipitation that percolated through the soil after 1950's atmospheric nuclear testing has not yet arrived to the water table and is believed to be in the vadose zone. Ground water with maximum tritium concentration (73 T U ) in the south is most likely derived from 1966/67 precipitation. For the calculation, we used the tritium decay curve and measured tritium concentrations in precipitation in Waco, Texas, and Albuquerque, New Mexico (International Atomic Energy Agency, 1971). Using the time period of 19 to 20 yr (1966/67 to 1986) and taking into account a short vertical path of 25 ft. (8 m) in the vadose zone (that is typical of the high-tritium zone), the annual recharge flux was calculated (Nativ, 1988, Appendix 5 ). The calculated recharge rate through the vadose zone ranges from 0.5 to 3.24 inches/yr (13 to 80 m m / y r ) , depending on the moisture content of the soil profile. It should be noted that this simplified model assumes a "piston-type" flow through a porous medium and the complete displacement of subsurface water in the vadose and saturated zones by the recharging water. This simplified model provides a minimum estimate for the water's age and a maximum estimate for annual recharge rate. (However, if vadose water moves through "shortcuts" such as cracks and joints, then the calculated recharge is too fast for a porous medium in which water movement is slower. ) Previous studies have estimated a slower diffusive recharge than the tritium-based recharge estimate (0.01 to 0.57 inch/yr or 0.3 to 15 mm/yr; Barnes, 1949; Klemt, 1981; Knowles et al., 1984; Stone, 1984; Stone and McGurk, 1985), whereas previous estimates for rates of recharge from playa lakes coincide better with the lower values of the tritium-based estimate (0.48 to 1.6 inches/yr or 12.2 to 41 m m / y r ; U.S. Bureau of Reclamation, 1982; Wood and Osterkamp, 1984; Stone, 1984; Stone and McGurk, 1985).

80 Based on the fast recharge rates that were calculated using tritium as a tracer and based on the slightly enriched values of $1sO and dD in ground water, we conclude that the most likely method for ground-water recharge is focused percolation of partly evaporated playa-lake water. A quantitative determination of each recharge component's contribution and the evaluation of the potential enrichment of ground water in the vadose zone requires more information about surface water and water at and above the capillary fringe. The problem can be addressed through volume and mass-balance measurements at playa lakes and analysis of soil cores (preferably adjacent to the monitored lakes ) for the distribution of chemical and isotopic composition with depth. ACKNOWLEDGEMENTS Precipitation was collected during 1984/85 by Charles Megee, Lawrence Smith, Jim Yates (National Weather Service stations in Lubbock, Amarillo and Midland, Texas, respectively), Jerry Antine ( K W K A - K T K M radio station in Clovis, New Mexico) and Ora Lee Frazier (Paducah, Texas). Charles Megee also assisted with the selection of Clovis and Paducah substations. At the Bureau of Economic Geology, The University of Texas at Austin, Alan R. Dutton, Bernd Richter, Harry H. Posey and S. Christopher Caran reviewed the manuscript and improved it significantly. Technical editing was by L.F. Brown, Jr. Drafting was by Marty T h o m p s o n under the supervision of Richard L. Dillon. Typesetting was by Rosanne M. Wilson and Dorothy C. J o h n s o n under the supervision of Lucille C. Harrell. The manuscript was edited by Mary Ellen Johansen. This work was supported by the Department of Energy Salt Repository Project Office. The conclusions of the authors are not necessarily endorsed or approved by the Department of Energy.

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