in this analysis. Second, the dutadon of 

 each of the three periods was not deemed 

 adequate to statistically appreciate the full 

 dimensions of these events. For example, 

 data for the flood event of 1967 (Period 

 II), which is the largest on record in the 

 past century, would have only been 

 considered within a 24. 1 -year span, and 

 would therefore have unacceptably skewed 

 the results of that period, especially during 

 the months in which the event occurred. 

 Accordingly, the omitted flow events for 

 Periods I, II and III were July 1942, 

 September-October 1967, and June-July 

 1987, respectively. ^\11 flow events, 

 however, were included in the analysis of 

 event timing, which is a more subjective 

 measure where the fiill dimensions of each 

 event are relevant for purposes of historical 

 comparison. 



c 

 o 



40 



30 - 



g. 

 o 



0) 



i_ 

 a. 



c 

 c 

 < 



n 20 



10 



1940-1957 1958-1981 1982-1999 



H Sabinal 



I I Corpus Christi 



19 Beeville 5 NE 

 □ Cotulla 



Figure 10: Mean annual precipitation of available data at 

 four gauges about the greater Nueces River watershed. 



Source: Medina 2000. Note: 1 inch = 2.54 cm 



It is interesting to note that Medina (2000) 



found the mean annual precipitation in the greater watershed during the first period (1940 through 1957) was 

 consistendy the smallest when compared with the other two periods (1958 through 1981, and 1982 through 

 1999) (Figure 10). The distribution of large precipitation events were also found to be less frequent during the 

 drovight years of the late 1940's and early 1950's than in the latter two periods, which were not significandy 

 different from each other (Medina 2000). This information is relevant because this first period, which was used 

 as the baseline for calculating percent changes in delta inflow, likely under-represents, to some degree, the actual 

 baseline conditions. 



Historical Freshwater Inflow into the Upper Nueces Delta 



Magnitude 



Historical event magnitude during Period I exhibited two primary peaks, one in the spring (May) and one in the 

 fall (September), each at over 43,173 10^ m' (35,000 acre-fit) per month (Figure 11). Dtxring Period 11, event 

 magtjitude also peaked twice during the year, but at lower discharge amounts. The spring peak Qune), which was 

 less defined than in Period I, attained only about 8,635 10^ m^ (7,000 acre-ft), while the fall peak (October) 

 attained almost 30,838 10^ m^ (25,000 acre-ft). During Period III, the trend of decreasing event magnitude was 

 observed in extreme. No annual peaks were obvious, and the highest monthly average discharge amount was 

 less than 370 10' m' (300 acre-ft) (May). 



Annual event magnitude represents the sum 

 of all daily discharge values during the 

 period divided by the period's duration in 

 years. During Period II, or after the 

 construction of Wesley Scale Dam, annual 

 event magnitude decreased by 39'yo 

 compared to Period I (Table 3). Since the 

 construction of Choke Canyon Dam 

 (Period III), the annual event magnitude of 

 discharge events into the upper Nueces 

 Delta decreased by over 99% from Period I. 



Table 3: Summary of historic annual event magnitude In the 

 upper Nueces Delta. Mean discharge values rounded to the nearest 

 10 acre-ft. 



Period 



1940-1958 

 1958-1982 

 1982-1999 



Historic 



Mean Total Discharge 



per Year 



(acre-ft) 



Percent Change 

 from Period I 



128,000 



78,000 



540 



-39% 

 -99.6% 



Note: 1 acre-ft = 1 2335 10^ m= 



C- 1 4 ^ Anafysis of the Historic Fhw Keffme of the Nueces River into the Upper Nueces Delta 



