Attempts to determine the hydrological balance observationally for the Amazon Basin as a whole, the world's largest rain forest ecosystem, have been hampered by uncertain estimates in each of the components of the hydrological balance: precipitation, interception, runoff, evaporation and advection. Moisture inflow from model reanalysis fails to account for as much as 50% of the estimated precipitation [Marengo, 2006]. Even in regions with a dense observational network as in the United States, the observational water balance fails to account for a large amount of water substance [e.g., Roads et al., 2002]. Precipitation is the most commonly measured component of the hydrological budget. Because of the patchy nature of convective rainfall, assembling regionally representative precipitation measurements remains a challenge. A significant systematic error in Amazon rainfall estimates could limit the utility of using the climate station record in assessing results of regional climate models, alter the forcing of ecosystem models [e.g., Botta et al., 2002], limit understanding of the role of drought in forcing ecosystem change, and impair assessments of the agricultural viability of the region.
 There is a clear systematic bias in the location of precipitation observing stations in the Amazon Basin. Since there are few roads in this region (Figure 1a), climate stations are near settlements along the rivers' banks (Figure 1b). Local mesoscale circulations near the 5–20 km wide rivers of the Amazon include the river breeze [Oliveira and Fitzjarrald, 1993, 1994; Silva Dias et al., 2004] and the ‘vegetation breeze', more frequently modeled than observed [D'Almeida et al., 2007]. The vegetation breeze, believed to result from circulations initiated by thermal contrasts at land cover change boundaries [Silva Dias et al., 2005; Ramos da Silva and Avissar, 2006] may promote cloudiness over cleared areas [Cutrim et al., 1995]. However, in areas where both effects are present, the much stronger water-land temperature contrasts mean that river breeze circulations should dominate any vegetation breeze.
 Daytime subsidence-induced clearing over the river with cloudiness inland is commonly observed in satellite images (Figure 1c) [Molion, 1987]. Molion and Dallarosa  examined data from four stations near Manaus for the period 1978–1988 and found that stations 100 km inland reported rainfall totals 20% higher than that observed at an island on the Negro River. They claimed similar rainfall depression observed at four stations near the Trombetas River near 56°W also resulted from breeze-induced subsidence, but in this case the explanation is not as convincing (see below). Other studies confirm that afternoon convective precipitation more typical of continental areas is observed inland from the river, with rainfall suppressed near the river [Ribeiro and Adis, 1984; Lloyd, 1990; Garstang and Fitzjarrald, 1999, p. 290; Cutrim et al., 2000]. The roughness difference between the river surfaces and the surrounding terrain causes the boundary layer wind to channel along the river course. We do not yet know whether proximity to the river influences the strength of nocturnal instability lines, a major source of precipitation in this region. Anecdotal evidence (e.g., Figure 1d) indicates that convection can even be enhanced along the Amazon River channel.
 For at least 45 years, it has been recognized that there is a ‘transverse dry zone' between Belém at the coast and Manaus [Reinke, 1962] (map reproduced as Figure 2 in the work of Haffer ) [Pires-O'Brien, 1997] in both the dry (July–December) and rainy (January–June) seasons. Recent rainfall estimates based on observations obtained using the satellite-based CMORPH technique [Joyce et al., 2004] confirm a sharp precipitation increase just to the west of 55°W (Figures 2a and 2b). The transverse dry zone just to the east of this longitude is clearly shown, and is particularly prominent at night during the rainy season. The pronounced rainy and dry seasons in the eastern Amazon Basin [Sombroek, 2001; Mahli and Wright, 2004] make its ecosystems particularly sensitive to perturbation by prolonged drought and fire [Nepstad et al., 2004]. In the face of extensive drought, some natural forests in the Amazon may be converted to savanna [Sternberg, 2001; Oyama and Nobre, 2003], a process that could be accelerated by deforestation associated with increasing intensive agriculture in recent years [Brown et al., 2005]. The presence of the El Niño reduces precipitation in the eastern Basin. The correlation between the Southern Oscillation index [Trenberth, 1984] and rainfall anomaly is largest near 55°W, the longitude of Santarém [Zeng, 1999; Liebmann and Marengo, 2001], but the correlation coefficient (≈0.6) is so small that other factors are needed to explain the bulk of the interannual variance. South Atlantic sea surface temperatures may also play a role [e.g., Ronchail et al., 2002; Marengo et al., 2008].
 Amazon rainfall reflects contributions both from convective systems stimulated by local forcing and from organized instability lines (referred to here as ‘squall lines') that move inland from the coast [Molion, 1987; Garstang et al., 1994; Cohen et al., 1995]. The tendency toward nocturnal wet season rainfall at the longitude of Santarém, evident in Figure 2b, has often been noted [Cutrim et al., 2000; Angelis et al., 2004; Moraes et al., 2005]. The evening precipitation preference at Santarém led Nechet  to describe the rainfall regime as ‘coastal', distinct from a typical inland pattern of afternoon rainfall [e.g., Lloyd, 1990].
 Squall lines arrive predominantly at night in Santarém, out of phase with afternoon heating. Molion  postulated that this explains the transverse dry zone between Belém and Manaus. Recent analyses of satellite-based rainfall sensors confirm that the instability lines propagating inland from the coast reach the longitude of Santarém at 03–04 UT [Negri et al., 2000; Kousky et al., 2005]. We can probably associate much of the nocturnal precipitation near 55°W with the nocturnal arrival of the squall lines and afternoon precipitation with local convective forcing. The squall lines arrive in Manaus (60°W) in the afternoon, in time to be in phase with afternoon locally forced convection [Garstang et al., 1994; Lloyd, 1990; Cutrim et al., 2000].
 The need to assess model output and calibrate remotely sensed data has led to gridded precipitation data sets based on rain gauge records [Liebmann and Allured, 2005; Xie and Arkin, 1996]. Because of their accessibility, such processed precipitation data are widely used for comparison with model output and used to assess (“validate”) remotely sensed data products. The considerable range of annual total Amazon Basin precipitation estimates [e.g., Costa and Foley, 1998] reflects inputs from differing data sources and the use of alternate methods to grid data [e.g., Willmott and Johnson, 2005]. The gridding techniques proposed to date do not account for the singular nature of the station proximity to the great rivers of the region. In addition, many approaches use reanalysis data, which is known to underestimate both moisture inflow and precipitation in the region, in part owing to the limited sounding network in the region [Marengo, 2006].
 The aims of this work are (1) to introduce a new detailed precipitation data set for the region of the eastern Amazon Basin near the Tapajós-Amazon river confluence, an important location near a strong regional precipitation gradient; and (2) to identify interannual, seasonal, diurnal, and spatial patterns in regional precipitation. Documenting rainfall patterns sets the stage to determine the extent to which river breezes or other mesoscale circulations may introduce a bias in the regional rainfall climate record. If river proximity effects are systematic and properly documented, more physically reasonable explanations of precipitation patterns, spatial correlations, and interpolation procedures can be adopted. Both instrument and sampling issues must be addressed. It seems unlikely that any bias would change the conclusions of the large-scale correlation studies, but it might be critical in issues that require quantitative accuracy, such as observationally closing the hydrological balance [Marengo, 2006].
 Do the influences of river breezes or other mesoscale effects lead to a systematic river proximity bias in Amazon rainfall data? We document the temporal and spatial patterns of precipitation in one region likely to be affected by river proximity. We constructed a mesoscale weather station network near Santarém, Pará Brazil (2°25′48″S, 54°43′12″W) as part of the Large Scale Biosphere-Atmosphere Experiment in Amazonia (LBA) Ecological Component (LBA-ECO) [Keller et al., 2004]. The geometry of rivers' confluence and the fact that ‘synoptic' and ‘diurnal' precipitation contributions can be easily distinguished according to the hour of occurrence make this region singularly useful to assess mesoscale landscape influences on precipitation. We analyze rainfall observed in this network collected during the period 1998–2006, and supplement the network with data from regional operational rain gauge networks for the same period and with satellite-based microwave sensor rain estimates. Analysis of errors associated with rain gauge sensors and their siting is given in section 2. At two sites, a long-term comparison of tipping bucket with conventional wedge gauges is used to assess measurement accuracy. Analysis of precipitation results for interannual, seasonal, and diurnal scales for the period 1998–2006 is presented in section 3. Conclusions and suggestions for continuing work (section 4) complete the paper. A study of other aspects of this study region with emphasis on observing the forcing mechanisms of the river breeze and its effect on other aspects of mesoclimate will appear in a companion paper.
1.2. Qualitative Predictions
 If river breezes generated by river-land temperature contrasts exert a dominant influence, stations near the river should be in a subsident region and lack an afternoon convective rainfall peak. One expects breezes to be more important during the dry season. The Amazon River is roughly parallel to the predominant easterlies, while the Tapajós is approximately normal. Mean low cloudiness is indeed suppressed over the rivers near the Tapajós-Amazon river confluence (Figure 1b). Naïve expectation is that the Tapajós breeze might produce the largest effect on precipitation since it lies roughly normal to the prevailing easterly winds, promoting enhanced convergence east of the river. This expectation was supported by model studies [Silva Dias et al., 2004; Lu et al., 2005], which correctly simulated enhanced cloudiness on the east bank of the Tapajós River for selected cases.
 Large instability lines propagate inland from the coast, arriving at Santarém (about 55° W) predominantly at night [Cohen et al., 1995; Kousky et al., 2005]. The nocturnal arrival time facilitates objective partition of rainfall into events of ‘basin scale' associated with these lines and those of local convective origin, a task previously done subjectively [e.g., Garstang et al., 1994]. One expects rainfall at all stations subject to predominantly squall rainfall should exhibit a nocturnal maximum. This has been taken to be the norm at the longitude of Santarém [e.g., Nechet, 1993; Angelis et al., 2004]. Since near-river stations would lack the afternoon convective component, one might expect a negative precipitation bias overall at stations near the rivers. Topographic effects would be expected to lead to increased rainfall along slopes oriented normal to the predominantly easterly regional flow. No simple qualitative prediction can be made from consideration of the channeling of the airflow over the rivers. Clear evidence of channeling exists for the mouth of the Amazon [e.g., Cohen et al., 2006]. We hypothesize that there is enhanced rainfall in convergent regions where the channel narrows after attaining a long trajectory over a wider water area, such as just to the west of the Amazon-Tapajós confluence.