Late last week, rain doused the Haitian capital of Port-au-Prince, heightening the dread of hundreds of thousands of people there who have been living in makeshift shelters since a massive earthquake destroyed great swaths of their city in January.

The rains start in earnest in April. And hurricane season arrives June 1. Cardboard and bed sheets—the materials that now serve as roofs and walls for countless people—are no match for Mother Nature. Even a plastic tarp will offer little comfort when the waters rush and rise. And they will.

This is Haiti where unchecked harvesting of wood—for construction, for charcoal–has left 98 percent of the country deforested, adding to the potential for flooding when heavy rain falls. And with many of the drainage channels around the capital now clogged with debris, where will the water go?

I’m remembering the anxious faces of the Haitians I met recently camped at Centre Sportif de Carrefour, a sports complex where several thousand homeless people had taken refuge under a variety of shelters, many of them constructed from sheets of white plastic stamped with “made in China” logos.

When it rains hard here, said Libermann Lexident, one of the camp leaders, the water pools up to three feet deep. That’s hip high on an adult. Everything below three feet gets soaked. Even so, he said, people would rather cope with the flooding than move back to their damaged homes, so profound is the fear the quake has left in its wake.

“If it’s raining, it’s going to be very hard,” said Lexident. “So far, we’ve been praying. It’s been answered. If it rains, we don’t know where to go.”

Last week’s downpour, drumming a warning on the plastic tarps strung across the capital, has heightened the urgency for tens of thousands of homeless families. Oxfam is distributing tents and plastic sheeting to thousands of them, and estimates indicate that there is enough shelter material in the capital, or en route, to meet the needs of about 50 percent of those who have been displaced. And aid groups think that as many as 40 percent of them could return to their homes if their buildings are declared safe. Oxfam has a team of structural engineers in the capital right now assessing that issue.

But as the rain approaches, the concern isn’t just for weather worthy shelter. Sanitation services have become a critical issue as well–especially latrines.

The numbers are frightening.

Aid groups estimate that the devastated region needs 18,000 toilets, but as the first-month anniversary of the quake approached, those groups and local workers had been able to dig fewer than 1,000 latrines. Oxfam had installed more than 20 percent of them—testament to its commitment in this area of expertise.

But the need remains enormous, especially as the rains approach and threaten to slop human waste into temporary settlements and crowded camps where there is little room to improve the drainage.

“We now need a surge in effort to improve sanitation facilities for people in Haiti,” said Marcel Stoessel, head of Oxfam in the country. “Let us not kid ourselves that this is going to be easy. It requires a Herculean humanitarian effort from all quarters. Around 230,000 people lost their lives on Jan. 12. It is our priority to make sure that we don’t let that number grow.”