Because of the heavy demand of water for agriculture and the insufficient flow of the San Joaquin River, groundwater in the San Joaquin Valley's rich aquifer has been an important source of irrigation supply since the late 19th century. Historically, surface water was able to provide all the needed supply, but as agricultural lands spread across more and more of the valley, groundwater pumping became increasingly common. Also, with the introduction of better technology that allowed farmers to dig deeper wells and install electrical pumps, groundwater was seen as an often cheaper and easily accessible source compared to the river. Water imported from Northern California through the Central Valley Project managed to stave off increasing rates of withdrawal for several years, until the 1977 California drought heavily decreased water supplies from these sources for a while and caused many farmers to return to groundwater pumping.