Hydrologic and material budgets for a small Sonoran Desert watershed during three consecutive cloudburst floods

'[oumalof Arid Environments (1985) 9, 105-118

Hydrologic and material budgets for a small Sonoran Desert watershed during three consecutive cloudburst floods S. G. Fisher* & N. B. Grimm" Accepted 28 October 1983 Precipitation and runoff chemistry were monitored during three successive summer storms on a small Sonoran Desert catchment. Both nitrogen and phosphorus concentrations in precipitation were high (1'38 and 0'21 mg/l respectively) yet lower than in runoff water (2'77 and 0'39 mg/l), However, 7590 per cent of precipitationdid not run off and the storm events represented net inputs of dissolved N and P to the watershed. Antecedentstorms influence both water chemistry and nutrient budgets and greatly deplete transportable particulates. Because precipitation in the Sonoran Desert is variable both temporally and spatially, caution should be excercised in attempting to construct 'typical year' watershed nutrient budgets. Despite low rainfall and sporadic runoff, hydrologic fluxes of nitrogen and phosphorus in desert watershedsare comparableto those of mesic regions. Chloride, H+, and SO~- were retained by the watershed during these storms while HC03" and dissolved and particulate organic matter were exported at rates higher than input by precipitation. Total dissolved solids entering in precipitation were three times export in stream flow while total particulate output greatly exceeded input in rain.

Introduction Summer precipitation in the Sonoran Desert results from a monsoon weather pattern which draws moisture from the Gulf of Mexico and, to a lesser extent, the Gulf of California (Sellers & Hill, 1974). Daytime heating generates convective cloudburst storms, usually in late afternoon or early evening in central Arizona. These storms are intense but of limited area, producing a patchy distribution of rainfall. Patches as small as a few kilometers in diameter may receive more than a centimeter of precipitation in a few minutes. Small, usually dry washes may experience flash floods during these precipitation events. Winter rains associated with westerly Pacific frontal storms are less intense and cover much larger areas. Annual precipitation in low deserts of the region ranges from 18 em (Phoenix) to 31 em (Tucson) while potential evapotranspiration (PE) is 10 times this. About half of annual precipitation and a somewhat larger fraction ofPE occurs in the warm season (May-September). Depending on the precipitation regime then, any given small wash may contain water and flow for only a few minutes to a few hours annually, or may not flow at all for several years. Nutrient inputs to small watersheds are precipitation, rock weathering, and certain biologic vectors (e.g. nitrogen fixation). Outputs occur as dissolved and particulate load • Department of Zoology, Arizona State University, Tempe, Arizona 85287, U.S.A. 0140-1963/85/050105

+ 14 $03.00/0

© 1985 Academic Press Inc.(London) Limited

S. G. FISHER & N. B. GRIMM

106

in streams draining these watersheds (Bormann & Likens, 1967). In mesic regions precipitation inputs are frequent and fluvial output relatively continuous, while in deserts these fluxes are rare and temporally restricted. As a result, few nutrient budgets exist for small watersheds of the desert south-west, although isolated flash floods have been described both physically and chemically (Ives, 1936; Schick, 1970; Fisher & Minckley, 1977). We report here both hydrologic and nutrient budgets for a small Sonoran Desert watershed which experienced cloudburst storms and subsequent runoff on three consecutive days in August 1982. This rare opportunity permits us to describe nutrient budgets of this little known ecosystem type, and to describe the effect of successive storms on these budgets. Methods and materials Study site

KR Wash drains a 0'65 ha watershed at 616 m elevation in western Pinal country, Arizona 10 krn east of Apache Junction near Kings Ranch (Fig. 1). The (33°23'N, 111 channel is first distinguishable 90 m from the top of the watershed (625 m elevation, 100 m from the sampling station. Channel slope is 4'2 per cent. Vegetation is typical of the lower Sonoran desertscrub life zone, and is dominated by the giant saguaro cactus (Carnegiea giganrea), jojoba (Simmondsia chinensis), bursage (Ambrosia deltoidea), creosote bush (Larrea tridentata), palo verde (Cercidium microphyllum), and ironwood (Olyneya 026'W)

20 m

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HYDROLOGIC AND MATERIAL BUDGETS DURING STORMS

107

tesotay. The watershed lies on an ancient outwash slope (bajada) of the Superstition Mountains. Soils are coarse, deep, and overlain with desert pavement.

Field and hydrologic techniques The three runoff events described here occurred on 22, 23, and 24 August 1982, the first two near 1700 hours and the last at 2300 hours. Precipitation was recorded with a rain gauge placed 30 m from the watershed. Precipitation for chemical analysis was collected with a polyethylene funnel-bottle combination at this same site. Stream discharge was estimated as the product of stream cross-sectional area and velocity, the latter measured by timed flotation. Water samples were taken at 2-5 min intervals through the course of the runoff event in clean polyethylene or glass(storm 2) containers. Samples were stored at 4 °C and filtered within 18 h. The watershed was mapped with a transit and area determined planimetrically. Watershed boundaries were identified based on topography and on direction of sheet flow during and immediately after storms. Chemical techniques and procedures Samples were filtered through pre-weighed Whatman GFIF® glass fiber filters after turbidity was determined with a Hach® model 2100 A turbidimeter. Suspended solids was determined gravimetrically after drying filters to constant weight at 95°C. Fine particulate organic matter (FPOM) was determined as loss on ignition (550°C, 2 h) of filtered materials. Samples were pre-filtered through 1 mm mesh netting, thus suspended solids and FPOM include only particles >0'7 ILm and <1 mm diameter. Conductivity and pH were measured with electrodes, chloride and alkalinity titrimetrically, and sulfate turbidimetrically (with BaClz). Dissolved organic matter (DaM) was measured by a dichromate oxidation technique (Maciolek, 1962) after evaporation of a 20-50 ml aliquot to dryness. Ammonium was measured by the phenolhypochlorite method (Solorzano, 1969). Nitrate was reduced to nitrate in cadmiumcopper columns (Wood, Armstrong et al., 1967) and nitrate determined colorimetrically after diazotization with sulfanilimide. Total dissolved nitrogen (TON) was determined as NH 3 + N0 3 after ultraviolet oxidation (Manny, Miller et al., 1971). Soluble reactive phosphorus (SRP) was measured colorimetrically by the molybdate-blue technique (Murphy & Riley, 1962) and total dissolved phosphorus (TOP) as SRP on ultraviolet oxidized samples. Dissolved organic nitrogen (DON) was taken as the difference between TON and (NH 3-N + N0 3-N); dissolved organic phosphorus (DOP) as TDP-SRP.

Results Hydrology Precipitation amounts on the three consecutive days were 14, 11 and 5 mm respectively and rainfall intensities were similar (Table 1). Stream flow duration approximately equalled storm duration, but was delayed 5-15 min. Runoff varied from 10'3-25 per cent of total precipitation and increased as a function of precipitation amount. Similarly, the first storm generated a maximum discharge of 27'2 lis compared with 20 lis in the second and 10 lis during the last and smallest storm. Ninety-nine per cent of water yield from the watershed occurred in just 18,20, and 12 min respectively in the three storms. Runoff associated with these isolated intense storms is thus equally intense and shortlived, yet 75-90 per cent of precipitation was retained by this small watershed.

S. G. FISHER & N. B. GRIMM

108

Table 1. Some hydrologic features of six late summer (1982) storms at KR Wash, Pinal Co., Arizona. Rise time isfrom first runoff to maximum discharge. Infiltration rate is measured in millimeters of H 20 retained perminute from first rainto lastrunoff Event date 22 Aug

23 Aug

24 Aug

7 Sept

11Sept (first event)

11Sept (second event)

Stormduration (min)

35

40

11

40

35

80

Precipitation (mm)

14

11

5

7

5

7

0'40

0'28

0'46

0'18

0'14

0'09

Flowduration (min)

36

36

22

0

0

0

Runoff(mm)

3'5

2'64

0'52

0

0

0

Discharge, max. (l/s)

27'2

20

10

0

0

0

Risetime(min)

5

14

6

0

0

0

Infiltration rate (mm/min)

0'22

0'17

0'17

0'18

0'14

0'09

25

24

10'3

0

0

0

Precipitation intensity (mm/min)

Percentage runoff

Precipitation chemistry Precipitation chemistry varied somewhat among storms (Table 2). Conductivity, an index of total dissolved ions, was highest during the first storm and declined to 20-23 fLS/cm during the latter two. Metallic cations were not determined due to limited sample volume; however, sulfate and chloride were the major inorganic anions in precipitation. In all storms, pH was low (4'5-5'1) and bicarbonate was thus rare or absent. Both nitrogen and phosphorus were remarkably abundant in precipitation samples. Ammonium and nitrate were the most abundant nitrogen species while dissolved organic phosphorus (DOP) was the predominant form of phosphorus except in the third storm when DOP was zero. DOM was low but present at 4'8-5'6 mg/l in all samples. Precipitation samples were not quantitatively analyzed for particulates; however, cursory examination of filters indicated the presence of trace amounts. Because the precipitation collector on the site is always open, chemical analyses include material leached from dry fall out. In addition, convective storms in the region are accompanied by high winds and entrained dust is also incorporated in rainfall. Variations in particulate content may help explain storm to storm variation in precipitation chemistry.

Runoffchemistry Chemistry of runoff water varied both among storms and during the course of individual runoff events. Measured concentrations of selected constituents during runoff are in Figs

HYDROLOGIC AND MATERIAL BUDGETS DURING STORMS

109

Table 2. Chemistry of bulk precipitation (P) and meanrunoff (RO) for storms of August 22, 23, and 24 1983 at KR Wash, Pinal Co., Arizona Storm 1 (22 August)

Storm 2 (23 August)

Storm 3 (24 August)

Parameter

P

RO

P

RO

P

RO

H20 (mm)

14

3'5

11

2'64

5

0'52

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4'5 32 0 1'0 6'5

64

61 6'4 0'4 5'7 4'5 8'4

23 5'1 8 0'6 3'4 2'0

46 6'8 0'2 7'6 3'7 3'0

20 4'5 32 0 2'3

*

73 6'7 0'2 7'3 6'7 5'7

NH~N(mg/l) NO~N (rng/l)

DON (rng/l) TDN (rng/l)

0'5 0'52 0'29 1'31

0'61 1'01 1'07 2'7

0'96 0'65 0'15 1'76

0'42 1'12 0'62 2'16

0'60 0'34 0'13 1-07

0'46 2'84 0'14 3'44

SRP (mg/l) DOP (mg/l) TDP (mg/l)

0'058 0'058 0'12

0'44 0'02 0'46

0'06 0'21 0'27

0'30 0'07 0'37

0'02

0'33 0'02 0'35

T T

1460 130 34'6

T T

250 38 30'2

T T

SS (mg/I) FPOM (mg/l) DOM (mg/l)

5'6

4'8

ND

0'02

5'1

470 54 43

• Sample lost, Abbreviations: ND, not detectable; T, trace amount, not measured; DON, dissolved organic nitrogen; TDN, total dissolved nitrogen; SRP, soluble reactive phosphorus; DOP, dissolved organic phosphorus; TDP, total dissolved phosphorus; SS,suspended solids; FPOM, fine particulate organic matter; DOM, dissolved organic matter. 2 and 3, while discharge-weighted means are included in Table 2. In general, materials in transport declined in concentration after an initial rise to maximum concentrations (e.g, suspended solids, FPOM). Other constituents exhibited highest concentrations in the first sample taken, despite the fact that discharge peaked some time later (e.g, HC0 3- , CI-, SRP). Only S04-S (flood 1) and DOM (flood 3) increased through the hydrograph; however, several constituents showed a slight rise in concentration as flow declined to zero (e.g. NOrN, SRP, conductivity). Most chemical constituents in runoff exceeded concentrations in precipitation. This was especially true of total and organic particulates, DOM, HC0 3- and SRP. [H+] declined markedly as water passed through the watershed and NHrN and DOP declined during two of the three storms. Between-storm variations in runoff chemistry are a bit obscure. All constituents except HC0 3- , NOrN and DOP declined between floods 1 and 2; however, many constituents rose again during flood 3. Notably, NOrN increased nearly three-fold from flood 1 to flood 3 and conductivity, chloride, DOM, and TON attained their highest concentrations in the last flood. It should be noted, however, that runoff during flood 3 was only 20 per cent of flood 2 and 15 per cent of flood 1. Nutrient budgets

Precipitation amount and chemical composition applied over the 0'65 ha watershed represents input for each constituent. Outputs in runoff are computed as the integral of

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HYDROLOGIC AND MATERIAL BUDGETS DURING STORMS

111

the product of concentration and discharge over time. Both inputs and outputs are presented as mass/area, (usually grams per hectare), for each storm event. Mass export (grams per hectare) divided by water output (e.g. liters per hectare) yields mean concentration for each constituent (Table 2). Comparison of inputs and outputs for each storm indicates whether the watershed represents a source or a sink of nutrients with respect to precipitation. In order for a given constituent to show a net loss during the precipitation-runoff event, mean concentrations in stream water must increase nearly four-fold during floods 1 and 2 and lO-foldduring flood 3, as water loss is just 25, 24, and 10 per cent of input respectively (Table 1). The watershed represented a source of SS, FPOM, DaM, HC0 3- and SRP in all storms, of TDP and DOP in storm 3 and chloride in storm 1 (Table 3). All other constituents were retained by the watershed as a consequence of these storm events; that is, input> output. Lowest fractional loss ratios (output/input) were exhibited by H+, NH 3-N, TDN and DOP (storms 1 and 2). Other elements were retained with ratios ranging from 0·1 to 0·99. Absolute export rates (kilogram per hectare) during each storm were a function of runoff volume and concentration of the constituent in that runoff. By far, most losses occurred as particulates. Suspended solids output during flood 1 reached 52 kg/ha, 4·6 kg/ha of which was organic (FPOM). Organic matter also represented the largest solution loss. DaM output during storm 1 was 1·2 kg/ha. No other single constituent was exported at a rate exceeding 1 kg/ha during any storm. Dissolved inputs via precipitation were generally higher than outputs with S04-S reaching 909 g/ha in storm 1. Total nitrogen inputs ranged from 54 to 192 g/ha, more than half of which was retained, and total phosphorus from 0·8 to 29 g/ha. Total dissolved solids (TDS) were not measured directly but can be estimated from conductivity. Assuming a conversion factor of 0'65 (Hem, 1970), TDS input ranged from 0'4 to 4 kg/ ha while output was 0'15-0·91 kg/ha. Mean TDS concentrations in precipitation were 42, 15 and 13 mg/l and in runoff, 40, 30 and 48 mg/l in the three floods, respectively. Dissolved losses ranged from 2·9 per cent (flood 1) to 10 per cent (floods 2 and 3) of total inorganic output, the remainder being particulate. Discussion The threshold amount of precipitation required to generate channel flow in arid regions is generally low. In the northern Negev desert, 1-2 mm of precipitation on rock and 3-5 mm on stony soil produced stream flow in small watersheds (Yair, Sharon et al., 1978). Surface abstractions by infiltration are small in compacted desert soils (Osborn & Renard, 1970) and overland flow quickly develops; however, depending upon watershed morphometry, this overland flow may not reach stream channels, as overland velocities of sheet flow are generally < 10 cm/s (Yair, Sharon et al., 1978). Out data indicate that precipitation intensity (millimeters per minute) must also be considered in predicting stream flow events. Three early September storms producing 5-7 mm precipitation failed to generate stream flowas precipitation intensities were all less than 0'2 mm/min; however, the 23 August storm (11 mm at 0·28 mm/min) and 24 August storm (5 mm at 0·46 mrn/min) yielded 2·6 and 0·52 mm runoff respectively (Table 1). Nahal Yael, a small watershed in the Negev Desert, typically produced channel runoff when >7'5 mm precipitation fell in 15 minutes (0·5 mm/min) (Schick 1970). In that study, initial infiltration rate was 0·17-0'5 mm/min; however, infiltratio~ drops rapidly to 10 per cent of initial rates in desert soils and surface runoff rapidly ensues (Osborn & Renard, 1970). We have no direct measures of infiltration in the KR Wash drainage, but based on water retained and elapsed time between onset of pre~ipitation and flow cessation, mean infiltration ranged from 0·17 to 0'22 mm/min dunng the three runoff events studied. Mean precipitation intensity of the three September storms which yielded no runoff was less than these estimated infiltration rates.

1'65 0'566 18'0 374 220

0'24 0'003

6'7 1'0 0'53

T T

+ +

52 4'6 1'2

T SS (kg/ha) FPOM (kg/ha) T DOM (kg/ha) 0'8 0'34

+, not calculated but very high.

Abbreviations: TDS, total dissolved solids,other acronyms as in Table 2.

1'5

8'0 1'9 9'9

6'1 22'9 28'9

1'91 0'1 1'0

15'5 0'8 16'3

8'1 8'1 16'3

11 30 16'5 58

SRP (g/ha) DOP (g/ha) TDP (g/ha)

16'5 194

72

105

0'31 0'5 0'94 0'54

22 39 38 99

NH,N(g/ha) 71 NO,N(g/ha) 73 DON (g/ha) 40'6 TDN (g/ha) 185

0'8 0'003 55'2 98 81

2'64

11

0'25

+

5'8 2'9 0 134 909

TDS (kg/ha) H+ (g/ha) HC03" (g/ha) CI- (g/ha) S04-S (g/ha)

3'5

Out

In

0/1

1'2 0'33

14

H 20 (mm)

Out

Storm 2

1'41 0'009 54'2 159 297

In

Parameter

Storm 1

1'6

2'4 0'28 0'14

T

0 0'17

1'71 0'09 1'80

0'8 T 0'8

1'32 0'08 0'34

+ +

2'4 14'6 0'74 17'7

30 17 6'5 54

0'11 0'42 1'0 0'30

0'52

Out

0'24 0'0006 9 34 29

5

In

0'65 1'03 0 117 212

0'49 0'005 3'1 0'26 0'37

0'24

0/1

Storm 3

+

+

0'8

+ +

2'25

2'14

0'08 0'86 0'11 0'33

0'29 0'14

0'38 0'0006

0'10

0/1

T T 1'31

15 31 46

206 162 63'6 433

8'1 4'5 18 625 1341

30

In

61 5'9 1'9

28

z-s

25'2

36 83'6 55'2 175

2'45 0'013 118 291 407

6'7

Out

All storms

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1'4

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1'68 0'09 0'61

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HYDROLOGIC AND MATERIAL BUDGETS DURING STORMS

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If precipitation exceeds infiltration capacity and persists long enough to generate sheet flow, channel flow will follow. At this point, small desert watersheds respond with rapidly increasing channel discharge as abstraction by channel sediments is negligible. In larger drainages, deep, porous sandy sediments store much of the channel flow, thus water yield decreases with watershed size (Osborn & Renard, 1970). In KR watershed, channel sediments are scarcely discernible from the soil surface and few channel deposits of sand exist. Our observations thus corroborate those of others describing storm runoff in desert streams: intense, short lived precipitation events, rapid exceedance of infiltration capacity, sheet then channel flow and rapid recession, the whole processes occurring in less than an hour. We agree with Fogel & Duckstein (1970) that antecedent rainfall has little effect on hydrologic responses ofsmall desert watersheds. Schick (1970) states that repeat storms greatly increase the likelihood of increased flood volumes if storms are merely hours apart. We observed no influence of sequential runoff-producing storms of August on each other, nor did the second storm of 2 September produce runoff although it occurred just 4 h after an earlier downpour. This relationship may require modification for larger watersheds with substantial channel storage capacity, however. Precipitation was collected as a bulk sample for each storm, thus chemical changes occurring through a given storm cannot be resolved. Dust is an important source of both phosphorous and nitrogen in precipitation and is diminished greatly through a storm event by atmospheric washout (Keup, 1968; Chapin & Uttormark, 1973). Total dissolved nitrogen in precipitation in this study ranged from 1'07 to 1'75 mg/l. Most of this was inorganic, with nitrate approximately equal to ammonia. These concentrations are threeto four-fold higher than comparable data from other western U.S.A. desert sites (West, 1978) and approach the high TDN value of 2'3 mg/l reported for precipitation on Ohio farmlands (Taylor, Edwards et al., 1971). Total dissolved phosphorus is also quite high, especially in storms 1 and 2 (0'12 and 0'27 mg/l) compared to Keup's (1968) reported range of 0'05-0'08 mg/l for several sites worldwide. Interpretation of within-flood chemical patterns is difficult, yet several patterns emerge. In nearly all cases, and most markedly with conductivity and all nitrogen and phosphorus species, highest concentrations occurred early in the runoff event. This is characteristic of ready solubilization of salts which have accumulated near the soil surface (Miller & Dever, 1977). Nitrate and chloride, having little adsorption capability, should respond markedly in this manner (Kurtz & Melsted, 1973); however, ammonia and phosphate (SRP) should be either more tightly bound or less soluble and respond gradually to increasing flow (Fuller, 1975; Tiedemann, Helvey et al., 1978). In our study, all of these ions showed similar early flood maxima suggesting low adsorption capacity of these desert soils. Rigler (1979) reports high particulate phosphorus concentrations on the ascending limb of the hydrograph and predominance of soluble P on the descending limb. The equilibrium between suspended particulate P and dissolved P may shift rapidly however (Taylor & Kunishi, 1977) and modify this pattern when suspended particulates are very high, such as in our study. Particulate materials (SS and FPOM) also showed a rapid rise in concentration at the onset of flooding. Easily eroded material was readily entrained with the first overland flow, and decreased subsequently. Interestingly, this phenomenon decreased in magnitude between floods 1 and 3 as the supply of erodable particulates was diminished. A slight rise in conductivity and concentration of several individual constituents occurred immediately prior to cessation of flow (Figs 2 and 3). Particulates did not show this; nitrate in storm 1 showed it dramatically. We suggest that this terminal rise is due to the longer contact time between this final runoff fraction and the soil compartment. The tail end of the hydrograph includes a disproportionate amount of water which fell as precipitation at the upper end of the watershed and which reached the gauging station last. In spite of this terminal rise in concentration, a marked clockwise hysteresis

114

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Figure 4. Hysteresis of dissolved materials (conductivity) and suspended solids during successive storms. Numbered points indicate minutes elapsed since onset of stream flow during storm 1. Clockwise hysteresis indicates higher concentrations on rising limb of hydrograph than at same discharge on falling limb. characterizes the concentration-discharge relationship for both total dissolved and total particulate constituents (Fig. 4). This difference between water chemistry at identical flows on rising and falling hydrograph limbs is consistent with temporal depletion of transportable materials and underscores the need for serial sampling to characterize floodassociated transport. Variation in runoff chemistry among storms may be explained in part by variation in flood magnitude, but is also influenced by antecedent storms. One effect of decrease in dilution volume was a higher conductivity of flood 3 runoff water. Among the Nand P species, only NOrN exhibited a greatly elevated concentration in flood 3 and at a fractional increase greater than any other chemical constituent. Virginia, Jarrell et al. (1982) report rapid denitrification following soil wetting at their Sonoran Desert site in California. Maximum rates occurred in just 22 h. Denitrification should result in a decrease in nitrate; however, ammonification in deserts is also a function of soil moisture (O'Brien, 1978) and subsequent nitrification of ammonia augments soil nitrate. While increased nitrification in response to wetting may take up to eight days in Great Basin Desert soils (Skuiins and Fulgham, 1978), our results suggest a more rapid accumulation of available nitrate in the Sonoran Desert. Input-output budget data illustrate several important points (Table 3). First, variance among storms was high. While several repeatable patterns occurred (e.g. high retention of [H +], high yield of particulates), some constituents showed net gains during one or more storms and net losses during others (e.g. Cr, TDP). As a result, great care should be exercised in constructing annual budget estimates from single-storm data. This is especially true when runoff is stochastic within a year and variable between years, as is typical of small desert catchments. Because annual budget data are so difficult to collect in deserts, comparison with ecosystems elsewhere is tenuous; however, since the three storms studied in detail here represent approximately 10 per cent of annual rainfall at the site (2·9 em of an annual 30·5 ern) we can roughly estimate annual fluxes by multiplying the sum of fluxes for the three storms by 10. This assumes that 23 per cent of precipitation appears in runoff annually, which may be an overestimate. Recall that three storms in September failed to produce any stream flow. In the year following the last of these, runoff at KR Wash occurred only four times.

HYDROLOGIC AND MATERIAL BUDGETS DURING STORMS

l1S

Our estimate of total annual solution output is 15'9 kg/ha and particulate output is 611 kg/ha/yr. While this estimated particulate loss is high compared to forested (25 kg/halyr) and clear-cut watersheds (190 kg/ha/yr) in New England (Likens & Bormann, 1974), it is considerably less than estimates for the Colorado River basin as a whole (4180 kg/halyr) (Judson & Ritter, 1964). This may be due in part to sporadic flow in small desert washes. Large floods with return frequencies of several years may push our estimate markedly upward. Estimated annual output of total dissolved nitrogen is 1'75 kglha which is at the low end of reported values for other ecosystems. In their summary of nitrogen output data for 17 small catchments at a variety of sites worldwide, Likens, Bormann et al, (1977) report an annual output range of 0'4-5'6 kglha. Seventy per cent of the inorganic nitrogen output at KR Wash is as N0 3-N which is typically the dominant form of inorganic nitrogen export from natural watersheds. Total dissolved phosphorus output (0'3 kg/halyr) is near the center of reported outputs elsewhere, which range from 0'02 kg/halyr on the Canadian shield (Kirchner, 1975) to 2'3 kg/halyr from the Great Ouse catchment in England (Owens, 1970). Inputs of nitrogen and phosphorus via precipitation are also within the range of values reported elsewhere; however NHrN slightly exceeds NOrN in precipitation at our site. We estimate total nitrogen input at 4'3 kg/ha/yr which compares with 1'1 kg/halyr in Ontario (Scheider, Snyder et aZ., 1979), and nearly 20 kg/halyr in both Ohio (Taylor, Edwards et aZ., 1971) and northern Britain (Owens, 1970). Our 0'46 kg/ha/yr total phosphorus input compares with 0'32 kg/halyr reported in Iowa (Jones, Borofka et al., 1976), 0'12 in Australia (Campbell, 1978), and 0'2-1'0 in northern Britain (Owens, 1970). Given low precipitation amounts in arid regions, even moderate Nand P inputs by this vector require high concentrations of these elements in precipitation. Average total dissolved nitrogen (TDN) in precipitation was indeed high (1'38 mg/l) as was TDP (0'14 mg/l), However, additional input occurs by dry fallout and in the case of nitrogen, by biological fixation. West (1978) estimates that nitrogen fixation in deserts may be between two and five times precipitation plus dry fallout inputs, yet suggests net gains are unlikely given high output rates via runoff, wind action, and denitrification. Our data show that both dissolved nitrogen and dissolved phosphorus are retained during summer storm events. Among nitrogen species, only 17 per cent of NHrN was exported, yet export of dissolved organic N was 87 per cent of precipitation input. About half of the N0 3-N entering in precipitation was retained in the catchment. Over 90 per cent of dissolved organic phosphorus was retained; however, soluble reactive phosphorus (largely orthophosphate) was exported at nearly twice the rate at which it entered. If particulate phosphorus is included in the budget and if suspended sediments are 0'25 per cent P (Rigler, 1979), then total phosphorus loss exceeded input by nearly four-fold. The nature of material budgets thus depends on the form in which the element occurs and different chemical combinations of the same element may move through the catchment quite differently. In addition to nitrogen and dissolved organic phosphorus, chloride, sulfate and hydrogen ion also were retained by the system (input> output). Precipitation pH was acidic and evidently was neutralized rapidly by the basic soils of the watershed. At the low pH of rain, bicarbonate does not occur, yet runoff water was high in HC0 3", again derived from alkaline soils. Both sulfate and chloride should be relatively inert biologically and in both cases, output was less than half of input. The increased concentration of both in runoff water indicates solubilization of soil salts; however, the fraction of sulfate and chloride in retained water adds to soil evaporites or percolates to deeper soil layers. Particulates exhibited a net loss from the catchment during the storm events, largely because of their low concentration in rain and ready transport in runoff. Dissolved organic matter also showed a net export even though rainfall concentrations were

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moderately high (4'8-5'6 mg/l). Exported DOM, presumably leached from accumulated soil organic matter, was approximately equal to particulate organic matter except during storm 1 when FPOM was very high. DOM did not show the depletion through the three storms that characterized FPOM. Our results have implications for both arid land watershed studies of nutrient cycling and for desert stream ecology. Watershed material budgets are useful in interpreting ecosystem-level processes in a variety of terrestrial system types. In arid regions, inputoutput budgets are difficult to construct because, rare, unpredictable, short-lived events are responsible for major fluxes of materials. Furthermore, these events must be studied in detail when they occur, with frequent runoff sampling and discharge measurements. Precipitation chemistry should also be measured periodically during a single storm to elucidate chemical changes occurring in discrete pulses of water which rapidly move through the small catchment. Finally, precipitation and runoff chemistry vary among storms and antecedent events greatly influence water chemistry. Year to year variation is also probably substantial and generalizations about typical years are hazardous without several year's data. While biological processes influence system chemistry between runoff events, dynamics of the short-lived event itself are probably largely physicalchemical - certainly more so than in heavily vegetated mesic watersheds. Aquatic biota is absent in these small watersheds, yet runoff from these catchments eventually feeds larger, more permanent streams and even larger desert rivers. We have found nitrate to be a key constituent influencing the metabolism of permanent streams in the Sonoran Desert (Grimm, Fisher et al., 1981). Nitrogen is often so low as to limit primary production of algae which represent the base of the aquatic food web. Only where water first emerges to the surface at main channel seeps or springs is nitrate high, and near these sites algal uptake is also substantial, but gradually depletes dissolved nitrogen in a downstream direction. Our results from KR Wash indicate that both precipitation and headwater runoff are quite high in nitrate (c. O'S and 1'7 mg NOrN/1 respectively). This water is then routed to channel storage in the extensive sandy sediments of downstream reaches. Its eventual emergence at spring heads supplies permanent reaches not only with nitrogen, but with a gradual release of water, which, along with winter runoff from higher elevations, sustains the system throughout the year. Permanent desert streams then, owe much of their chemical and biological nature to fleeting cloudburst storms which may have occurred miles and months away on rocky desert landscapes. This workwassupportedin part bya NationalScience Foundationgrant DEB 8004145 to S. Fisher.

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