Spaniards who explored and settled the Great Plains from the sixteenth through the early nineteenth centuries brought with them a set of legal and cultural values about water that had been fostered over the centuries in Spain and later in central Mexico. The arid and semiarid stretches of northern New Mexico, southern Colorado, and western Texas, which constituted the far northern frontier of New Spain, shaped settlement patterns as colonists sought water to sustain rural economic activities. Access to, and control over, this natural resource influenced the rhythms of daily life and not infrequently precipitated conflict between individuals and communities. As a result, knowledge and application of Spanish water rights became the purview of farmers, lawyers, judges, and irrigators.

Spanish water law in America was heir to a long and dynamic legal tradition that began in Spain under the Roman Empire and continued after subsequent invasions by Germanic and Muslim peoples. In 1265, Alfonso X ordered the codification of disparate legal codes and customs and, although elements of Germanic and Islamic law can be found in the compilation, the Siete Partidas was the adaptation of Roman Law to a medieval Spanish reality. With a few modifications and additions over the years, the Siete Partidas remained the primary legal code that governed water rights. After the Spanish conquest of Mexico and Peru in the sixteenth century, the general principles of Spanish jurisprudence were given a distinctly American context in a legal code written specifically for Spain's vast empire. In 1681, the Spanish crown promulgated the Recopilación de las leyes de los reynos de las Indias, constituting the foundation of Spanish water law in America. It vested original ownership of all land, water, and minerals in the Spanish Crown, which was empowered to alienate from its domain these resources to private individuals, towns, and informal rural communities.

Spanish jurisprudence recognized two kinds of property rights that are fundamental to understanding Spanish water law. Surface water was propiedad imperfecta, which meant that it was subject to qualification and measured against the rights of others. The Spanish civil law of property did not recognize riparian rights to running streams or rivers. If a piece of property fronted on a stream, for example, the owner could only use the water for domestic purposes. The Spanish Crown did convey surface water rights, however, for agricultural or industrial purposes through a specific grant (merced de agua). On the far northern frontier of New Spain a judicial procedure known as repartimiento de aguas often was used to convey such rights. But it was more common for surface water to be granted automatically if the land classification itself reflected a conveyance (e.g., if a plot of land was designated as tierras de pan llevar, or irrigable land, then the subsequent grant carried water rights). Groundwater, however, was propiedad perfecta. Ownership of spring water and rainwater was nearly absolute, and landowners could not be easily deprived of these waters once conveyance was extended, even if use of it caused damage to neighbors.

Three social entities with juridical personality competed for water on Spain's far northern frontier: Spanish towns, Indian pueblos, and informal rural communities known today as acequia (ditch) associations. Judges who presided over water disputes allocated resources according to seven principles: just title, prior use (not the same as prior appropriation in Anglo common law), need, legal right, intent, noninjury to third party, and equity and the common good. The dry expanse of the Great Plains fashioned a legal atmosphere that eschewed water monopolies, and judges applied a combination of these principles to facilitate community harmony and social tranquility. Even after Mexico's independence from Spain in 1821, judges continued to employ these principles as Spanish water law maintained its full vigor.

Moreover, Spanish water law survives today in the Great Plains because the Treaty of Guadalupe Hidalgo, which ended hostilities between the United States and Mexico in 1848, obliged the American government to respect the property of those Mexicans who suddenly found themselves residing north of the newly created boundary. Since water was considered property under Spanish and Mexican law, the U.S. courts act as surrogates for Hispanic water law.

See also HISPANIC AMERICANS: Guadalupe Hidalgo, Treaty of; Hispano Homeland.