0 10 20 30 MILES

EXPLANATION

TEXAS

Coastal Bend

 study area

LOCATION MAP

Data-collection site

NUECES

RIVER

FRIO

RIVER

CHOKE

 CANYON

 RESERVOIR

ATASCOSA

 RIVER

NUECES RIVER

LAKE

 CORPUS

 CHRISTI

SAN

DIEGO

CREEK

SANTA

GERTRUDIS

JA

CREEK BONCILLO S

CREEK

LOS

OLMOS

CREEK

SAN

FE RNAN DO

CREEK

PETRONILA

CREEK

BAFFIN

BAY

ST. CHARLES

 BAY

COPANO

 BAY

ARANSAS

NUECES BAY CORPUS

 CHRISTI

 BAY

GULF OF MEXICO

OSO BAY

COPANO CR.

MISSION

RIVER

97o

97o 98 30' o

30'

98o

28o30'

28o

27o

27o30'

McMULLEN

LIVE OAK

BEE

DUVAL

JIM WELLS

NUECES

KLEBERG

BROOKS KENEDY

GOLIAD

SAN PATRICIO

REFUGIO

ARANSAS

LAGUNA MADRE

GULF OF MEXICO

RIVER

KINGSVILLE

ROBSTOWN

PORT ARANSAS

ARANSAS

 PASS

PORTLAND

CORPUS

 CHRISTI

MATHIS

EDROY 1 INGLESIDE

2

2

National Weather Serv

 rain gage

(), 

community-based effort to identify the status of problems associ

p, , 

quality, sediments, and living resources within the Co

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact S

Figure 1. Location of selected data-collection sites and National Weather Service rain gages, Coastal Bend area, south Te
2

Atmospheric deposition, resulting from rainfall and dry deposition of atmospheric particles, has been identified nationwide as a 

significant nonpoint source of nutrients to surface waters either 

from direct deposition on surface water or from stormwater runoff. Baird and others (1996, p. 72) reported that 4.8 percent of the 

total nitrogen deposited on Oso Bay during 1988–92 originated 

from rainfall directly onto the bay. Quenzer and others (1998, 

p. 84) reported that atmospheric sources account for about onethird of the total nitrogen deposited on the area bays and estuaries.

The Coastal Bend study area in south Texas (fig. 1) has a 

population of about 540,000 (Texas State Data Center, 1999), 

with about one-half residing in Corpus Christi, the major urban 

center in the area. Other cities in the Coastal Bend area include 

Aransas Pass, Ingleside, Kingsville, Portland, and Robstown. 

Land use in the Coastal Bend study area is about two-thirds 

agriculture and ranching. Corpus Christi is a major port and 

home to the Nation’s third largest refinery and petrochemical 

complex. The bays and estuaries in the area are known for their 

recreational and commercial fisheries. The climate of the Coastal 

Bend area is subtropical (short, mild winters and long, hot and 

humid summers). Prevailing winds are southeasterly most of the 

year but are briefly interrupted during winter and spring by 

southward-moving fronts. Average annual rainfall varies from 

about 25 to 35 inches (in.). The mean monthly and annual rainfall 

during 1967–96 for three National Weather Service (NWS) rain 

gages in the Coastal Bend area (Corpus Christi, Kingsville, and 

Mathis) are listed in table 1.

During 1996–98, the U.S. Geological Survey (USGS), in 

cooperation with the Texas Agricultural Experiment Station in 

Corpus Christi, Tex., collected and analyzed rainfall for nitrogen 

at two sites in the Coastal Bend area. This fact sheet presents 

concentrations of several forms of nitrogen from 39 rainfall events 

at the two sites. It describes the computation of nitrate nitrogen, 

ammonia nitrogen, and total nitrogen deposition from the rainfall 

and concentration data; and presents the regression equations 

from which daily deposition was estimated. Monthly and annual 

deposition obtained by summing daily deposition also are 

presented.

y y 

San Patricio County (fig. 1). The southern site (site 2

Kingsville in northern Kleberg County. Rainfall data 

lected using automatic samplers equipped with polye

buckets. Rainfall at each site was recorded by a tippin

rain gage measuring 0.01-in. increments. At the end o

event, the samples were retrieved and shipped to the 

National Water Quality Laboratory in Arvada, Colo., 

analysis. The samples were analyzed for dissolved ni

gen, dissolved nitrite nitrogen, dissolved ammonia ni

ammonia plus organic nitrogen, and dissolved ammo

organic nitrogen.

Samples were collected and analyzed for 39 rainfa

the two sites during May 1996–October 1998. During 

collection period, site 1 had a total of 79.02 in. of rai

had a total of 71.58 in. The samples collected at sites 

resent 28.71 and 29.20 in. of rain, respectively (table 

amounts correspond to 36 and 41 percent of rainfall t

site during the sampling period. Sampled rainfall eve

from 0.19 to 5.52 in.; the mean and median rainfall a

1.48 and 1.03 in., respectively.

Nitrogen concentrations from the water samples c

represent rainfall event-mean concentrations (EMC). 

tests were done to determine if EMCs significantly d

between samples collected at the two data-collection 

Wilcoxon rank-sum test (Helsel and Hirsch, 1992) w

test whether differences between median concentratio

nitrate nitrogen, ammonia nitrogen, and total nitrogen 

two sites were significant. The tests did not indicate s

differences (p-values for nitrate, ammonia, and total n

0.14, 0.65, and 0.67, respectively). As a result of the 

from both sites were combined into one dataset for fu

analysis.

Summary statistics for dissolved nitrate nitrogen, 

nitrite nitrogen, dissolved ammonia nitrogen, total an

ammonia plus organic nitrogen, and total nitrogen (ta

computed for the combined dataset. Total nitrogen w

lyzed directly but was computed from other analyses

cases, total nitrogen was computed as the sum of disso

dissolved nitrite, and total ammonia plus organic nitr

ever, for several samples, concentrations of dissolved 

nitrogen were greater than concentrations of total am
3

1 Laboratory reporting limit is 0.1 mg/L. Values less than 0.1 mg/L are laboratory estimates.

08/08/96 1.08 5.7 2.8 .15 <.001 .13 <.1 .01 

08/09/96 .31 -- -- .22 <.001 .21 .30 .30 

08/21/96 1.12 5.4 -- .11 .002 .17 .17 .20 

08/24/96 1.21 5.5 -- .07 <.001 .08 .14 .05 

09/10/96 .50 -- -- .32 .003 .38 .50 .40 

09/20/96 .56 -- -- .21 <.001 .23 .30 .30 

11/24/96 .79 -- -- .04 <.001 .16 .20 .20 

12/15/96 .68 -- -- .22 <.001 .30 .80 .80 

02/12/97 .21 -- -- .67 <.001 .75 .80 .80 

03/11/97 .41 6.0 2.8 .10 <.01 .18 .30 .20 

05/09/97 1.31 -- -- .22 <.001 .23 .16 .19 

06/22/97 4.48 -- -- .10 .001 .10 .04 .02 

09/21/97 .78 -- -- .17 .005 .17 .29 .20 

10/06/97 .25 -- -- .14 <.001 .09 .14 .16 

10/11/97 3.43 -- -- .02 <.001 .02 .08 .04 

10/18/98 5.27 -- -- .24 .005 .10 .10 .17 

Site 2

05/11/96 .47 -- -- .17 <.01 .41 .50 .40 

08/09/96 .19 -- -- .22 <.001 .24 .30 .30 

08/21/96 .61 6.2 -- .20 .002 .14 .15 .17 

08/23/96 .90 -- -- .06 .002 .20 .40 .16 

09/20/96 1.63 -- -- .07 <.001 .20 .20 .30 

11/24/96 .28 -- -- .11 <.001 .32 .50 .50 

12/15/96 .31 -- -- .14 <.001 .30 .40 .30 

03/11/97 .65 7.8 9.2 .10 .01 .14 .30 .30 

04/02/97 1.73 -- -- .03 <.001 .06 <.1 .01 

04/03/97 3.51 -- -- .03 <.001 .09 .01 .01 

05/09/97 1.03 4.9 -- .17 <.001 .18 .14 .13 

05/16/97 2.02 -- -- .12 .002 .15 .08 .08 

08/23/97 .40 -- -- .45 .01 .09 .45 .21 

09/22/97 2.74 -- -- .06 .002 .11 .13 .10 

09/24/97 1.96 -- -- .10 .001 .28 .28 .26 

10/11/97 5.52 -- -- .04 <.001 .01 .14 .12 

02/14/98 1.54 -- -- .14 .002 .30 .38 .34 

09/23/98 1.50 5.2 -- .07 .001 .05 .09 .01 

10/18/98 2.21 6.0 -- .06 .001 .05 .09 .04 
4

0123456

RAINFALL, IN INCHES

0.0

0.1

0.2

NITRATE AND AMMO

organic nitrogen1. In those cases, total nitrogen was computed as 

the sum of dissolved nitrate, dissolved nitrite, and dissolved 

ammonia nitrogen. Rain-weighted mean concentrations for dissolved nitrate nitrogen, dissolved ammonia nitrogen, and total 

nitrogen were computed by summing the product of r

EMC for the 39 rainfall events and dividing by the su

rainfall.

Concentrations

Total nitrogen EMCs ranged from 0.09 to 1.5 mill

liter (mg/L), with an arithmetic mean of 0.42 mg/L (t

Concentrations of nitrate and ammonia nitrogen were 

in all samples and account for most (about 81 percen

total nitrogen detected in the samples. Nitrite concen

 1 The analysis for dissolved ammonia nitrogen is more precise than 

the analysis for total (or dissolved) ammonia plus organic nitrogen at 

concentrations less than 0.1 mg/L. The laboratory reporting limit for 

dissolved ammonia nitrogen is 0.002 mg/L and for total (or dissolved) 

ammonia plus organic nitrogen is 0.1 mg/L.

Figure 2. Event-mean concentrations of nitrate and ammonia nitrogen, Coastal Bend area, south Texas, 1996–98.

Table 3. Summary statistics of event-mean concentrations of nitrogen from rainfall, Coastal Bend area, south Texas, 1996–

[mg/L, milligrams per liter; --, not determined; <, less than] 

Constituent No. of

samples

Arithmetic

mean

Rain-weighted

mean Median Standarderror of mean

Standard

deviation Minimum 

Nitrogen, nitrate, dissolved 

(mg/L as N)

39 0.15 0.11 0.14 0.019 0.12 0.02 

Nitrogen, nitrite, dissolved 

(mg/L as N)

39 -- -- <.001 -- -- <.001 

Nitrogen, ammonia, dissolved 

(mg/L as N)

39 .19 .12 .17 .022 .14 .01 

Nitrogen, ammonia plus organic, 

total (mg/L as N)

39 .25 .15 .20 .031 .19 <.1 

Nitrogen, ammonia plus organic, 

dissolved (mg/L as N)

39 .23 .15 .20 .029 .18 .01 

Nitrogen, total (mg/L as N) 39 .42 .28 .35 .048 .28 .09 
5

0123456

RAINFALL, IN INCHES

0.00

0.05

0.10

TOTAL NITRO

equations (Helsel and Hirsch, 1992, p. 256–257). The 

regression equations for deposition (table 4) were of 

L = a • Rb

 • Φ,

where

L = estimated deposition, in pounds per acre;

a, b = regression coefficients;

R = rainfall, in inches; and

Φ = bias-correction factor.

A comparison between total nitrogen deposition c

the rainfall events and estimated from the regression 

functions of rainfall, is shown in figure 3.

Estimated monthly and annual total nitrogen depo

1996–98 at the two data-collection sites, computed b

daily deposition estimated from the regression equati

listed in table 5.

greater than the laboratory reporting limit were detected in 15 of 

the 39 samples (38 percent) but never greater than 0.01 mg/L. 

Organic nitrogen (not analyzed directly, but calculated as the 

difference between ammonia plus total organic nitrogen and dissolved ammonia) was detected in most samples and accounts for 

about 19 percent of the total nitrogen. EMCs of nitrate and ammonia nitrogen decreased with increasing rainfall amounts (fig. 2).

Deposition

An EMC multiplied by the rainfall, in inches, and a conversion 

factor yields the deposition (pounds per acre) for the rainfall 

event. Deposition for nitrate nitrogen, ammonia nitrogen, and total 

nitrogen for the 39 rainfall events were computed accordingly. 

Least-squares regression was used with the computed eventdeposition amounts to develop relations between deposition and 

rainfall for nitrate nitrogen, ammonia nitrogen, and total nitrogen. 

The resulting equations were used with daily rainfall totals 

(measured at the sampling site) to estimate daily deposition for 

unsampled rainfall events; estimated daily depositions were 

summed to determine monthly and annual deposition.

Because of the nonlinearity observed in the relations between 

deposition and rainfall amounts, the data were log-transformed 

for regression. The estimated depositions from log-transformed 

regression equations, when detransformed to original units, are 

biased low and thus underestimate the actual depositions. To 

compensate, bias-correction factors equal to the mean of the 

regression residuals (in original units) were applied to the 

Figure 3. Comparison of computed and estimated (from regression) total nitrogen deposition for selected rainfall events, C

area, south Texas, 1996–98.

Table 4. Regression equations for estimating nitroge

from rainfall, Coastal Bend area, south Texas, 1996–9

Nitrogen form Deposition eq

Nitrate, dissolved 0.025 • R0.433 

Ammonia, dissolved 0.034 • R0.372 

Total 0.076 • R0.456 
6

Monthly rainfall nitrogen deposition was highly variable, 

ranging from 0 to 0.72 pound per acre (September 1998, site 2). 

Greater annual rainfall generally resulted in greater deposition. 

However, site 1 deposition in 1997 was greater than site 2, even 

though site 2 had greater annual rainfall because, compared with 

site 1, more of the rainfall at site 2 occurred in larger events2

 when 

nitrogen concentrations were lower (fig. 2).

References

Baird, C.F., Dybala, T.J., Jennings, Marshall, and Ockerman, D.J., 

1996, Characterization of nonpoint sources and loadings to 

the Corpus Christi Bay National Estuary Program study area: 

Corpus Christi Bay National Estuary Program, CCBNEP–05, 

226 p.

Helsel, D.R., and Hirsch, R.M., 1992, Studies in environmental 

science 49—Statistical methods in water resources: 

Amsterdam, Elsevier, 522 p.

Quenzer, A.M., Maidment, D.R., Hellweger, F.L., Eid, N.J., Ward, 

G.H., and Armstrong, N.E., 1998, Total loads and water 

quality in the Corpus Christi Bay System: Corpu

National Estuary Program, CCBNEP–27, 133 p.

Texas State Data Center, 1999, Total population estim

for Texas counties and places: Department of Ru

Sociology, Texas Agricultural Experiment Statio

Texas A&M University, accessed May 4, 1999, a

http://txsdc.tamu.edu/txpop97.html.

—D.J. Ockerman3

 and C.W. Livingston4

3 U.S. Geological Survey.

4 Texas Agricultural Experiment Station.

Any use of trade, product, or firm names is for descri

purposes only and does not imply endorsement by the 

U.S. Government.

For more information, please contact:

Subdistrict Chief

U.S. Geological Survey

435 Isom Road, Suite 234

San Antonio, Texas 78216

Email: gbozuna@usgs.gov

Phone: (210) 321–5200

Fax: (210) 530–6008

World Wide Web: http://tx.usgs.gov/

 2 During 1997, daily rainfall of 0.01 in. or more occurred 101 days at 

site 1 compared to 77 days at site 2. Site 1 had an average daily rainfall 

(considering only days when rainfall occurred) of 0.33 in. The site 2 daily 

rainfall was 0.46 in.

1998

1 31.87 .12 .37 .27 .08 0 .05 .09 .55 .36 .61 .34 .26 

2 28.87 .11 .26 .19 .04 0 .05 .04 .38 .72 .55 .37 .15