udf density depending on depth
 

hi everyone, I am new in udf and I haven't any information about it. I want to define density that depend on depth of ocean. density with going down the ocean change like the picture. I need yo define this changing of density with changing depth. anyone can help me to write this udf?
p.s. : function of changing is not important. I just want code for desity depend on depthhttp://aquarius.umaine.edu/images/ed_DENSVSDP.jpg