Precipitation patterns

Patterns of precipitation amount and variability in Amazonia change with both latitude and longitude. Analysis of TRMM data shows average annual precipitation in the Amazonian lowlands peaks at ~2-4°S latitude, with a broad plateau from ~4°N to 4° S of the equator. Precipitation decreases steadily to the south and north (Figure 10.3e). Precipitation at low latitudes can be either high or low, while sites to the north and south have more uniformly low rainfall. Precipitation variability, measured as dry-season length and total variability, reaches a minimum between ~1°N and 4°S (Figures 10.3a and 100.3c, respectively). Large areas of upper Amazonia have almost no variability in rainfall from month to month and year to year. Near the equator at Yasuni, Ecuador, maximum monthly rainfall has occurred in every month of the year (Pitman et al., 2001). Precipitation variability increases as one moves away from the equator, with dry season length increasing through 20°S, and precipitation variability reaching a peak ^15° S (Figures 10.3a, c).

In contrast to latitude, mean annual precipitation shows little relationship to longitude, though areas of high rainfall are more common in western Amazonia, causing a slight decrease in average rainfall as one moves from west to east (Figure 10.3f). Dry season length is on average shorter in western Amazonia and increases to the east, though the main cause of the trend is the absence of areas with no dry season in eastern Amazonia (Figure 10.3d). Total variability in rainfall is lowest in western Amazonia and increases consistently as one moves east (Figure 10.3b).

Latitude And Longitude Detroit

LATITUDE LONGITUDE

Figure 10.3. Latitudinal and longitudinal variation in Amazonian precipitation variability (a) and (b), dry-season length (c) and (d), and total amount (e) and (f) derived from TRMM measurements. Average dry-season length and precipitation variability reach a minimum centered at ~2-3°S latitude. Dry-season length and precipitation variability are lowest in western Amazonia and incease to the east. Precipitation also peaks ~2-3°S, but shows little trend from west to east.

LATITUDE LONGITUDE

Figure 10.3. Latitudinal and longitudinal variation in Amazonian precipitation variability (a) and (b), dry-season length (c) and (d), and total amount (e) and (f) derived from TRMM measurements. Average dry-season length and precipitation variability reach a minimum centered at ~2-3°S latitude. Dry-season length and precipitation variability are lowest in western Amazonia and incease to the east. Precipitation also peaks ~2-3°S, but shows little trend from west to east.

As noted by other authors, precipitation variables are correlated (e.g., Gentry, 1988; Clinebell et al., 1995; Ter Steege et al., 2003). Mean dry-season length decreases with increasing rainfall, though areas with the highest rainfall can have dry-season lengths that vary from 0 to 4 months (Figure 10.4a). Total variability in precipitation is on average lowest in areas with high rainfall and increases with decreasing rainfall down to ~ 1,800 mm yr-1, below which it is approximately constant (Figure 10.4b). At precipitation levels above ^ 1,800 mm yr-1, however, one can find sites with either high or low total variability in precipitation. Variability among years shows little trend with annual precipitation above 1,500 mm yr-1, save for a slight tendency for areas

Tropical Rainforest Precipitation

1000 2000 3000 2 6 10

Figure 10.4. Amazonian rainfall and three measures of variation in rainfall derived from 7 years (1998-2004) of satellite measurements from the Tropical Rainfall Monitoring Mission (TRMM). (a) Total precipitation versus length of dry season as measured by months having <100-mm precipitation (Spearman's rho, rs — —0.85). (b) Total variability in rainfall (measured as the coefficient of variation) among 94 months of precipitation versus accumulated rainfall (rs — 0.60). (c) Inter-annual variability versus accumulated rainfall (rs — —0.17). (d) Total variability versus dry-season length (Spearman's rho, rs — 0.86). In all cases measures of variability in precipitation decrease with increasing rainfall. There is substantial variability in the trend, however, with sites at any given precipitation having short or long dry seasons, and predictable or unpredictable rainfall amounts from month to month or year to year.

1000 2000 3000 2 6 10

Rainfall (mm yr-1) Months <100 mm

Figure 10.4. Amazonian rainfall and three measures of variation in rainfall derived from 7 years (1998-2004) of satellite measurements from the Tropical Rainfall Monitoring Mission (TRMM). (a) Total precipitation versus length of dry season as measured by months having <100-mm precipitation (Spearman's rho, rs — —0.85). (b) Total variability in rainfall (measured as the coefficient of variation) among 94 months of precipitation versus accumulated rainfall (rs — 0.60). (c) Inter-annual variability versus accumulated rainfall (rs — —0.17). (d) Total variability versus dry-season length (Spearman's rho, rs — 0.86). In all cases measures of variability in precipitation decrease with increasing rainfall. There is substantial variability in the trend, however, with sites at any given precipitation having short or long dry seasons, and predictable or unpredictable rainfall amounts from month to month or year to year.

with high annual precipitation to have fewer sites with high inter-annual variability. Total variability in precipitation is highly correlated with dry-season length (rs — 0.80), but the increase in variability with dry-season length is slight until dry seasons exceed 5 months in length.

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