Atmospheric and hydrological water balance

The hydrological cycle of the Amazon region is of great importance since the region plays an important role in the functioning of regional and global climate. Variations in its regional water and energy balances at year-to-year and longer timescales are of

Tocantins River

Figure 9.3. Seasonal cycle of river discharges and levels in northern and southern Amazonia. Levels are in cm (Rio Negro at Manaus). Discharges from the Amazon, Madeiras, and Tocantins Rivers are in 103 m3 s_1. Source: M. Costa and M. Coe.

Rfcr Negro

Tocantins River

JFMAM.tJASOND

Madeira River

Figure 9.3. Seasonal cycle of river discharges and levels in northern and southern Amazonia. Levels are in cm (Rio Negro at Manaus). Discharges from the Amazon, Madeiras, and Tocantins Rivers are in 103 m3 s_1. Source: M. Costa and M. Coe.

AmtsiOfi Rirtr

AmtsiOfi Rirtr special interest, since alterations in circulation and precipitation can translate ultimately to changes in the streamflow of the Amazon River. In addition, these changes can also affect atmospheric moisture transport from the Amazon region to adjacent regions. Since the late 1970s, large-scale water budget studies have been conducted for this region using a variety of observational data sets varying from radiosondes to global reanalyses (Salati, 1987; Matsuyama, 1992; Eltahir and Bras, 1994; Marengo et al., 1994; Rao et al., 1996; Vorosmarty et al., 1996; Costa and Foley, 1999; Curtis and Hastenrath, 1999; Zeng, 1999; Labraga et al., 2000; Roads et al., 2002; Marengo, 2004b and references quoted therein). Most of these reanalyses discuss the impacts of remote forcing on variability of the components of water balance, as well as the role of evapotranspiration in water balance.

The lack of continuous precipitation and evaporation measurements across the entire basin and of measurements of river discharge along the Amazon River and its main tributaries has forced many scientists to use indirect methods for determining the water balance for the region. Early studies by Salati and Marques (1984) attempted to quantify the components of the water balance by combining observations from the few radiosonde stations in Amazonia and models to estimate evapotranspiration. The Amazon Rivers drains an area of approximately 5.8 x 106 km2, with an average discharge of 5.5 x 1012m3yr_1. However, different estimates of the area of the Amazon Basin by different authors have led to a wide range of computed discharges of the Amazon River during the last 25 years. Most of these estimates are based on the records of the Amazon at Obidos, and are shown in Table 9.1. The differences are due to the different areas considered, and more recently (Marengo, 2004b) the discharges at Obidos (available since the mid-1970s) were corrected by the Brazilian National Water Agency ANEEL so that they would be more representative of the discharge at the delta of the Amazon River.

Results of previous studies of the annual water budget in Amazonia are listed in Table 9.2 (compiled from Matsuyama, 1992; Marengo et al., 1994; Costa and Foley, 2000; Marengo, 2004b). Main differences in results are due to the different areas considered for the basin that translate to different discharge and derived run-off, different precipitation networks and methods of assessment (mostly based on gridded rainfall data or from rain gauges distributed irregularly in the basin), and the methods used to determine annual water balance, where evapotranspiration ET is estimated as residual precipitation P and discharge R. However, the equation P = ET + R does not guarantee accurate estimates of the possible role of the tropical forest in recycling moisture for rainfall.

The annual cycle of water balance terms shows some differences between northern and southern sections of the basin (Figure 9.3). There is seasonality in R and P: with R peaking between 3 and 4 months after P. The E/P ratio of the dry season is larger than that of the rainy season, indicating that the role of evaporation (and evapotranspiration) on the water cycle is relatively more important in the dry season than in the rainy season. The largest E/P is found during the dry season in the southern region, reaching values greater than 1, which is larger than respective values in the northern region (Marengo, 2004b). In general, in the present climate the Amazon Basin can be considered as a moisture sink (P > E).

Table 9.1. Observed river discharge for the Amazon River at Obidos (Matsuyama, 1992; Marengo et al., 1994; Marengo and Nobre, 2001; Marengo, 2004b).

Study

Amazon River discharge (103m3 s-1)

Leopold (1962)

113.2

UNESCO (1971)

150.9

Nace (1972)

175.0

UNESCO (1974)

173.0

Baumgartner and Reichel (1975)

157.0

Villa Nova et al. (1976)

157.0

Milliman and Meade (1983)

199.7

Nishizawa and Tanaka (1983)

160.0

Oki et al. (1995)

155.1

Matsuyama (1992)

155.1

Russell and Miller (1990)

200.0

Vorosmarty et al. (1989)

170.0

Sausen et al. (1994)

200.0

Marengo et al. (1994)

202.0

Costa and Foley (1998a)

162.0

Zeng (1999)

205.0

Leopoldo (2000)*

160.0

Leopoldo (2000)**

200.0

Roads et al. (2002)

224.0

Marengo (2004b)*

175.0

Marengo (2004b)**

210.0

* Measured at Obidos.

** Measured (corrected) at the mouth.

** Measured (corrected) at the mouth.

Estimates of the water balance in the Amazon region exhibit some uncertainities, derived mainly from the use of different rainfall data sets (either gridded or station data), use of streamflow data at the Obidos gauge site (corrected or uncorrected), and use of global reanalyses produced by some meteorological centers in the U.S. and Europe. The National Centers for Environmental Prediction (NCEP) and the European Center for Medium Range Weather Forecast (ECMWF) have carried out retrospective analyses (reanalyses) over the last decade using a single model and data assimilation to represent climate evolution from as early as World War II. These reanalyses can highlight characteristic features of circulation and water balance. However, while data assimilation should in principle provide for a description of the water flux field, there are no guarantees that this description will be superior to that obtained from objective analysis and radiosonde observations alone, especially over continental regions. There is a need for the level of uncertainty to be identified in the measurement or estimation of the components of the water budget.

Table 9.3 shows the annual values of the water budget components for Amazonia in its entirety, giving the mean and two extremes of interannual variability: El Nino

Table 9.2. The annual water budget of the Amazon Basin. P = Precipitation; ET = Evapotranspiration; R = Streamflow—all in mmy-1. In this table the water balance equation ET = P — R is used, P and R are measured, and ET is obtained as a residual. Marengo (2004b) used the water balance equation that considers the non-closure of the Amazon Basin (Marengo and Nobre, 2001; Marengo, 2004b).

Table 9.2. The annual water budget of the Amazon Basin. P = Precipitation; ET = Evapotranspiration; R = Streamflow—all in mmy-1. In this table the water balance equation ET = P — R is used, P and R are measured, and ET is obtained as a residual. Marengo (2004b) used the water balance equation that considers the non-closure of the Amazon Basin (Marengo and Nobre, 2001; Marengo, 2004b).

Study

P

ET

R

Baumgartner and Reichel (1975)

2,170

1,185

985

Villa Nova et al. (1976)

2,000

1,080

920

Jordan and Heuveldop (1981)

3,664

1,905

1,759

Leopoldo et al. (1982)

2,076

1,676

400

Franken and Leopoldo (1984)

2,510

1,641

869

Vorosmarty et al. (1989)

2,260

1,250

1,010

Russell and Miller (1990)

2,010

1,620

380

Nishizawa and Koike (1992)

2,300

1,451

849

Matsuyama (1992)

2,153

1,139

849

Marengo et al. (1994)

2,888

1,616

1,272

Costa and Foley (2000)

2,166

1,366

1,800

Marengo (2004b)

2,117

1,570

1,050

1982-83, and El Nino 1997-98 and La Nina 1988-89. Reduced precipitation (P), runoff (R), and moisture convergence (C) are found during these two strong El Nino events while values larger than normal are found during La Nina 1988/89, and in all cases P > E, suggesting that the Amazon region is an atmospheric moisture sink. The difference between these two El Nino events in Amazonia is that during 1997/98 large-scale circulation anomalies over the Atlantic sector did not allow for much convergence of moisture. In the long term P > E, during La Nina events it is shown that P > E, and during El Nino 1982-83 and 1997/98 P > E, even though the difference is smaller than the mean and La Nina years.

Table 9.3. Climatological water budget 1970-99 for the Amazon Basin. Comparisons are made for the 1982-83 El Nino and the 1988-89 La Nina. P is derived from observations (Marengo, 2004b), E and C are derived from NCEP/NCAR reanalyses, and R is run-off from the historical discharge records of the Amazon River at Obidos. Units are in mmday—1. +C = Moisture convergence (Marengo, 2004b).

Table 9.3. Climatological water budget 1970-99 for the Amazon Basin. Comparisons are made for the 1982-83 El Nino and the 1988-89 La Nina. P is derived from observations (Marengo, 2004b), E and C are derived from NCEP/NCAR reanalyses, and R is run-off from the historical discharge records of the Amazon River at Obidos. Units are in mmday—1. +C = Moisture convergence (Marengo, 2004b).

Component

Mean

El Nino 1982-83

El Nino 1997-98

La Nina 1988-89

P

5.8

4.6

5.2

6.7

E

4.3

4.5

4.1

4.4

R

2.9

2.1

2.5

2.9

C

1.4

1.3

1.3

3.1

P - E

+1.5

+0.4

+0.9

+2.3

P - E - C

+0.1

-0.9

-0.1

-0.8

Imbalance = [((C/R) - 1)]

51%

38%

52%

6%

Table 9.4. Water budget 1970-99 of the entire Amazon Basin. P is derived from several data sources: Global Historical Climatology Network (GHCN), Xie and Arkin (CMAP), GPCP, NCEP, Legates-Wilmott (LW), Climate Research Unit (CRU) and from observations by Marengo (2004a). E and C are derived from NCEP/NCAR reanalyses, and R — Corrected run-off from the historical discharge records of the Amazon River at Obidos. Units are in mmday-1. +C — Moisture convergence (Marengo, 2004b).

Component GHCN CMAP GPCP NCEP LW CRU Marengo

Table 9.4. Water budget 1970-99 of the entire Amazon Basin. P is derived from several data sources: Global Historical Climatology Network (GHCN), Xie and Arkin (CMAP), GPCP, NCEP, Legates-Wilmott (LW), Climate Research Unit (CRU) and from observations by Marengo (2004a). E and C are derived from NCEP/NCAR reanalyses, and R — Corrected run-off from the historical discharge records of the Amazon River at Obidos. Units are in mmday-1. +C — Moisture convergence (Marengo, 2004b).

Component GHCN CMAP GPCP NCEP LW CRU Marengo

(2004b)

P

8.6

5.6

5.2

6.4

5.9

6.0

5.8

E

4.3

4.3

4.3

4.3

4.3

4.3

4.3

R

2.9

2.9

2.9

2.9

2.9

2.9

2.9

C

1.4

1.4

1.4

1.4

1.4

1.4

1.4

P -

E

4.3

1.3

0.9

2.1

1.6

1.6

1.5

P -

E- C

+2.9

-0.1

-0.5

+0.7

+0.2

+0.3

+0.1

The rain gauge based rainfall estimates used by Marengo (2004b) produced a mean of 5.8mmday—\ which is close to the values obtained in similar studies using different rainfall gridded data sets (CMAP, CRU, GHCN, and GPCP). The observed R at the mouth of the Amazon has been estimated as 2.9 mm day"1 (or 210,000 m s for a basin area of 6.1 million square kilometers), and this represents the combination of Amazon discharges at Obidos and those of the Xingu and Tocantins Rivers. Table 9.4 shows that, depending on the rainfall observational data set used, the results for water balance in the region can vary and the P — E difference can get as high as 4.3 mm day"1 (GHCN) or as low as 0.9mmday—1 (GPCP).

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  • matias
    What is the role the hydrologic cycle plays in tropical rain forest?
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