Inconsistent Farnham gave a lethargic display and crashed to defeat at lowly Merstham who showed a greater desire to win.
The home side also adapted better to the condititions. Farnham tried to play a passing game, but often got bogged down, whereas Merstham were much more effective with their direct route to goal.
With the heavy pitch soon cutting up, Farnham got off to the worst possible start, conceding a goal with barely a minute gone.
Town failed to clear their lines after Eldred had played a corner short to Hearn and the ball fell to Giwa who scored from close range.
On 16 minutes, Gareth Slessor was unlucky not to equalise with a long-range shot that hit the crossbar and bounced over.
If that had gone in, the outcome could have been different. As it was, Merstham added a second goal on 25 minutes to effectively seal the points.
Williams booted the ball upfield and Nigel Moore, under pressure, lost possession to Mumbi who ran clear to beat Chris Edmonds in the Farnham goal.
Town had chances of their own before half-time, but tended to over-elaborate instead of shoot. Chris Tomlinson came closest with a header that was cleared off the line by Eldred.
In the second half, Farnham found themselves forced back by a more determined Merstham who were only denied further goals by the reliable Edmonds.
The visitors had their best spell in the last 10 minutes, when Graeme Miles and Alan Morgan both brought good saves from McBride.
Farnham: Edmonds, Wheeler, Slessor, Moore, Decicco, Tomlinson, Wisdom (Edwards), Raby, Miles, Morgan, Lloyd (Annetts). Sub (not used) Middleton.
Farnham are home to Cove this Saturday (ko 3 pm).