The Collaborative International Dictionary of English v.0.48:

Lysimeter \Ly*sim"e*ter\ (l[-i]*s[i^]m"[-e]*t[~e]r), n. [Gr.
   ly`sis a loosing + -meter.]
   An instrument for measuring the water that percolates through
   a certain depth of soil. --Knight.
   [1913 Webster]